diff --git a/library/DataDefs.cpp b/library/DataDefs.cpp
index 28761efa3..f376edc6a 100644
--- a/library/DataDefs.cpp
+++ b/library/DataDefs.cpp
@@ -159,6 +159,14 @@ enum_identity::enum_identity(size_t size,
     }
 }
 
+enum_identity::enum_identity(enum_identity *base_enum, type_identity *override_base_type)
+    : enum_identity(override_base_type->byte_size(), base_enum->getScopeParent(),
+                    base_enum->getName(), override_base_type, base_enum->first_item_value,
+                    base_enum->last_item_value, base_enum->keys, base_enum->complex,
+                    base_enum->attrs, base_enum->attr_type)
+{
+}
+
 enum_identity::ComplexData::ComplexData(std::initializer_list<int64_t> values)
 {
     size_t i = 0;
@@ -455,18 +463,22 @@ void DFHack::flagarrayToString(std::vector<std::string> *pvec, const void *p,
     }
 }
 
-static const struct_field_info *find_union_tag_candidate(const struct_field_info *fields, const struct_field_info *union_field)
+static const struct_field_info *find_union_tag_candidate(struct_identity *structure, const struct_field_info *union_field)
 {
     if (union_field->extra && union_field->extra->union_tag_field)
     {
         auto defined_field_name = union_field->extra->union_tag_field;
-        for (auto field = fields; field->mode != struct_field_info::END; field++)
+        for (auto p = structure; p; p = p->getParent())
         {
-            if (!strcmp(field->name, defined_field_name))
+            for (auto field = p->getFields(); field && field->mode != struct_field_info::END; field++)
             {
-                return field;
+                if (!strcmp(field->name, defined_field_name))
+                {
+                    return field;
+                }
             }
         }
+
         return nullptr;
     }
 
@@ -476,32 +488,32 @@ static const struct_field_info *find_union_tag_candidate(const struct_field_info
         name.erase(name.length() - 4, 4);
         name += "type";
 
-        for (auto field = fields; field->mode != struct_field_info::END; field++)
+        for (auto p = structure; p; p = p->getParent())
         {
-            if (field->name == name)
+            for (auto field = p->getFields(); field && field->mode != struct_field_info::END; field++)
             {
-                return field;
+                if (field->name == name)
+                {
+                    return field;
+                }
             }
         }
     }
 
-    if (name.length() > 7 &&
-            name.substr(name.length() - 7) == "_target" &&
-            fields != union_field &&
-            (union_field - 1)->name == name.substr(0, name.length() - 7))
-    {
-        return union_field - 1;
-    }
-
-    return union_field + 1;
+    return nullptr;
 }
 
-const struct_field_info *DFHack::find_union_tag(const struct_field_info *fields, const struct_field_info *union_field)
+const struct_field_info *DFHack::find_union_tag(struct_identity *structure, const struct_field_info *union_field)
 {
-    CHECK_NULL_POINTER(fields);
+    CHECK_NULL_POINTER(structure);
     CHECK_NULL_POINTER(union_field);
 
-    auto tag_candidate = find_union_tag_candidate(fields, union_field);
+    auto tag_candidate = find_union_tag_candidate(structure, union_field);
+
+    if (!tag_candidate)
+    {
+        return nullptr;
+    }
 
     if (union_field->mode == struct_field_info::SUBSTRUCT &&
             union_field->type &&
diff --git a/library/LuaTypes.cpp b/library/LuaTypes.cpp
index 740b3f689..ef69958e0 100644
--- a/library/LuaTypes.cpp
+++ b/library/LuaTypes.cpp
@@ -616,6 +616,16 @@ static int meta_struct_index(lua_State *state)
     if (!field)
         return 1;
     read_field(state, field, ptr + field->offset);
+    if (field->mode == struct_field_info::SUBSTRUCT || field->mode == struct_field_info::CONTAINER)
+    {
+        auto struct_type = (struct_identity*)get_object_identity(state, 1, "read", false);
+        if (auto tag_field = find_union_tag(struct_type, field))
+        {
+            get_object_ref_header(state, -1)->tag_ptr = ptr + tag_field->offset;
+            get_object_ref_header(state, -1)->tag_identity = tag_field->type;
+            get_object_ref_header(state, -1)->tag_attr = field->extra ? field->extra->union_tag_attr : nullptr;
+        }
+    }
     return 1;
 }
 
@@ -631,6 +641,16 @@ static int meta_struct_field_reference(lua_State *state)
     if (!field)
         field_error(state, 2, "builtin property or method", "reference");
     field_reference(state, field, ptr + field->offset);
+    if (field->mode == struct_field_info::SUBSTRUCT || field->mode == struct_field_info::CONTAINER)
+    {
+        auto struct_type = (struct_identity*)get_object_identity(state, 1, "reference", false);
+        if (auto tag_field = find_union_tag(struct_type, field))
+        {
+            get_object_ref_header(state, -1)->tag_ptr = ptr + tag_field->offset;
+            get_object_ref_header(state, -1)->tag_identity = tag_field->type;
+            get_object_ref_header(state, -1)->tag_attr = field->extra ? field->extra->union_tag_attr : nullptr;
+        }
+    }
     return 1;
 }
 
@@ -679,6 +699,81 @@ static int meta_struct_next(lua_State *state)
     return 2;
 }
 
+/**
+ * Metamethod: iterator for unions.
+ */
+static int meta_union_next(lua_State *state)
+{
+    if (lua_gettop(state) < 2) lua_pushnil(state);
+
+    int len = lua_rawlen(state, UPVAL_FIELDTABLE);
+    int idx = cur_iter_index(state, len+1, 2, 0);
+    if (idx == len)
+        return 0;
+
+    auto header = get_object_ref_header(state, 1);
+    if (header->tag_ptr)
+    {
+        if (idx != 0)
+            return 0;
+
+        auto enum_id = (enum_identity*)header->tag_identity;
+        auto tag_val = *(int64_t*)header->tag_ptr;
+        size_t tag_shift = 64 - 8 * enum_id->byte_size();
+        tag_val <<= tag_shift;
+        tag_val >>= tag_shift;
+
+        size_t tag_index = tag_val - enum_id->getFirstItem();
+        if (auto complex = enum_id->getComplex())
+            tag_index = complex->value_index_map.count(tag_val) ? complex->value_index_map.at(tag_val) : size_t(-1);
+
+        if (tag_index >= size_t(enum_id->getCount()))
+            return 0;
+
+        const char *tag_name = nullptr;
+        if (header->tag_attr)
+        {
+            for (auto enum_field = enum_id->getAttrType()->getFields(); enum_field->mode != struct_field_info::END; enum_field++)
+            {
+                if (!strcmp(enum_field->name, header->tag_attr))
+                {
+                    if (enum_field->type == df::identity_traits<const char*>::get())
+                    {
+                        auto attrs = ((uint8_t*)enum_id->getAttrs()) + (tag_index * enum_id->getAttrType()->byte_size());
+                        tag_name = *(const char **)(attrs + enum_field->offset);
+                    }
+                    break;
+                }
+            }
+        }
+        else
+        {
+            tag_name = enum_id->getKeys()[tag_index];
+        }
+
+        if (!tag_name)
+            return 0;
+
+        lua_getfield(state, UPVAL_FIELDTABLE, tag_name);
+        if (lua_isnil(state, lua_gettop(state)))
+        {
+            lua_pop(state, 1);
+            return 0;
+        }
+
+        lua_pop(state, 1);
+        lua_pushstring(state, tag_name);
+        lua_getfield(state, 1, tag_name);
+
+        return 2;
+    }
+
+    lua_rawgeti(state, UPVAL_FIELDTABLE, idx+1);
+    lua_dup(state);
+    lua_gettable(state, 1);
+    return 2;
+}
+
 /**
  * Field lookup for primitive refs: behave as a quasi-array with numeric indices.
  */
@@ -806,6 +901,25 @@ static int check_container_index(lua_State *state, int len,
     return idx;
 }
 
+static void attach_container_item_tagged_union(lua_State *state, int container, int item, int idx)
+{
+    auto header = get_object_ref_header(state, container);
+    if (header->tag_ptr)
+    {
+        // TODO: handle bit vector for tag
+
+        auto tag_container = (container_identity*)header->tag_identity;
+
+        auto ref = get_object_ref_header(state, item);
+
+        // on both msvc and gcc, vectors have the same memory layout
+        auto item_type = tag_container->getItemType();
+        ref->tag_ptr = (*(uint8_t**)header->tag_ptr) + size_t(idx) * item_type->byte_size();
+        ref->tag_identity = item_type;
+        ref->tag_attr = header->tag_attr;
+    }
+}
+
 /**
  * Metamethod: __index for containers.
  */
@@ -820,6 +934,7 @@ static int meta_container_index(lua_State *state)
     int len = id->lua_item_count(state, ptr, container_identity::COUNT_READ);
     int idx = check_container_index(state, len, 2, iidx, "read");
     id->lua_item_read(state, 2, ptr, idx);
+    attach_container_item_tagged_union(state, 1, -1, idx);
     return 1;
 }
 
@@ -837,6 +952,7 @@ static int meta_container_field_reference(lua_State *state)
     int len = id->lua_item_count(state, ptr, container_identity::COUNT_WRITE);
     int idx = check_container_index(state, len, 2, iidx, "reference");
     id->lua_item_reference(state, 2, ptr, idx);
+    attach_container_item_tagged_union(state, 1, -1, idx);
     return 1;
 }
 
@@ -873,6 +989,7 @@ static int meta_container_nexti(lua_State *state)
 
     lua_pushinteger(state, idx);
     id->lua_item_read(state, 2, ptr, idx);
+    attach_container_item_tagged_union(state, 1, -1, idx);
     return 2;
 }
 
@@ -1194,7 +1311,6 @@ static void IndexFields(lua_State *state, int base, struct_identity *pstruct, bo
         lua_pop(state, 1);
 
         bool add_to_enum = true;
-        const struct_field_info *tag_field = nullptr;
 
         // Handle the field
         switch (fields[i].mode)
@@ -1208,16 +1324,11 @@ static void IndexFields(lua_State *state, int base, struct_identity *pstruct, bo
             continue;
 
         case struct_field_info::POINTER:
-            // Skip class-typed pointers within unions and other bad pointers
-            if ((pstruct->type() == IDTYPE_UNION || (fields[i].count & 2) != 0) && fields[i].type)
+            // Skip potentially bad pointers
+            if ((fields[i].count & 2) != 0 && fields[i].type)
                 add_to_enum = false;
             break;
 
-        case struct_field_info::SUBSTRUCT:
-        case struct_field_info::CONTAINER:
-            tag_field = find_union_tag(fields, &fields[i]);
-            break;
-
         default:
             break;
         }
@@ -1229,17 +1340,6 @@ static void IndexFields(lua_State *state, int base, struct_identity *pstruct, bo
         if (add_to_enum)
             AssociateId(state, base+3, ++cnt, name.c_str());
 
-        if (tag_field)
-        {
-            // TODO: handle tagged unions
-            //
-            // tagged unions are treated as if they have at most one field,
-            // with the same name as the corresponding enumeration value.
-            //
-            // if no field's name matches the enumeration value's name,
-            // the tagged union is treated as a structure with no fields.
-        }
-
         lua_pushlightuserdata(state, (void*)&fields[i]);
         lua_setfield(state, base+2, name.c_str());
     }
@@ -1275,7 +1375,8 @@ void LuaWrapper::IndexStatics(lua_State *state, int meta_idx, int ftable_idx, st
  * Make a struct-style object metatable.
  */
 static void MakeFieldMetatable(lua_State *state, struct_identity *pstruct,
-                               lua_CFunction reader, lua_CFunction writer, bool globals = false)
+                               lua_CFunction reader, lua_CFunction writer,
+                               lua_CFunction iterator, bool globals = false)
 {
     int base = lua_gettop(state);
 
@@ -1287,7 +1388,7 @@ static void MakeFieldMetatable(lua_State *state, struct_identity *pstruct,
     IndexFields(state, base, pstruct, globals);
 
     // Add the iteration metamethods
-    PushStructMethod(state, base+1, base+3, meta_struct_next);
+    PushStructMethod(state, base+1, base+3, iterator);
     SetPairsMethod(state, base+1, "__pairs");
     lua_pushnil(state);
     SetPairsMethod(state, base+1, "__ipairs");
@@ -1434,7 +1535,15 @@ void bitfield_identity::build_metatable(lua_State *state)
 void struct_identity::build_metatable(lua_State *state)
 {
     int base = lua_gettop(state);
-    MakeFieldMetatable(state, this, meta_struct_index, meta_struct_newindex);
+    MakeFieldMetatable(state, this, meta_struct_index, meta_struct_newindex, meta_struct_next);
+    SetStructMethod(state, base+1, base+2, meta_struct_field_reference, "_field");
+    SetPtrMethods(state, base+1, base+2);
+}
+
+void union_identity::build_metatable(lua_State *state)
+{
+    int base = lua_gettop(state);
+    MakeFieldMetatable(state, this, meta_struct_index, meta_struct_newindex, meta_union_next);
     SetStructMethod(state, base+1, base+2, meta_struct_field_reference, "_field");
     SetPtrMethods(state, base+1, base+2);
 }
@@ -1442,7 +1551,7 @@ void struct_identity::build_metatable(lua_State *state)
 void other_vectors_identity::build_metatable(lua_State *state)
 {
     int base = lua_gettop(state);
-    MakeFieldMetatable(state, this, meta_struct_index, meta_struct_newindex);
+    MakeFieldMetatable(state, this, meta_struct_index, meta_struct_newindex, meta_struct_next);
 
     EnableMetaField(state, base+2, "_enum");
 
@@ -1464,7 +1573,7 @@ void other_vectors_identity::build_metatable(lua_State *state)
 void global_identity::build_metatable(lua_State *state)
 {
     int base = lua_gettop(state);
-    MakeFieldMetatable(state, this, meta_global_index, meta_global_newindex, true);
+    MakeFieldMetatable(state, this, meta_global_index, meta_global_newindex, meta_struct_next, true);
     SetStructMethod(state, base+1, base+2, meta_global_field_reference, "_field");
     SetPtrMethods(state, base+1, base+2);
 }
diff --git a/library/LuaWrapper.cpp b/library/LuaWrapper.cpp
index 05c4db789..59bd96732 100644
--- a/library/LuaWrapper.cpp
+++ b/library/LuaWrapper.cpp
@@ -171,6 +171,9 @@ void LuaWrapper::push_object_ref(lua_State *state, void *ptr)
     auto ref = (DFRefHeader*)lua_newuserdata(state, sizeof(DFRefHeader));
     ref->ptr = ptr;
     ref->field_info = NULL;
+    ref->tag_ptr = NULL;
+    ref->tag_identity = NULL;
+    ref->tag_attr = NULL;
 
     lua_swap(state);
     lua_setmetatable(state, -2);
diff --git a/library/include/DataDefs.h b/library/include/DataDefs.h
index f4eff8510..174156e76 100644
--- a/library/include/DataDefs.h
+++ b/library/include/DataDefs.h
@@ -239,6 +239,7 @@ namespace DFHack
                       const char *const *keys,
                       const ComplexData *complex,
                       const void *attrs, struct_identity *attr_type);
+        enum_identity(enum_identity *enum_type, type_identity *override_base_type);
 
         virtual identity_type type() { return IDTYPE_ENUM; }
 
@@ -332,6 +333,8 @@ namespace DFHack
                 struct_identity *parent, const struct_field_info *fields);
 
         virtual identity_type type() { return IDTYPE_UNION; }
+
+        virtual void build_metatable(lua_State *state);
     };
 
     class DFHACK_EXPORT other_vectors_identity : public struct_identity {
@@ -842,7 +845,7 @@ namespace DFHack {
      * As a special case, a container-type union can have a tag field that is
      * a bit vector if it has exactly two members.
      */
-    DFHACK_EXPORT const struct_field_info *find_union_tag(const struct_field_info *fields, const struct_field_info *union_field);
+    DFHACK_EXPORT const struct_field_info *find_union_tag(struct_identity *structure, const struct_field_info *union_field);
 }
 
 #define ENUM_ATTR(enum,attr,val) (df::enum_traits<df::enum>::attrs(val).attr)
diff --git a/library/include/DataIdentity.h b/library/include/DataIdentity.h
index 88a96f9f3..711712af7 100644
--- a/library/include/DataIdentity.h
+++ b/library/include/DataIdentity.h
@@ -578,7 +578,7 @@ namespace df
 
 #ifdef BUILD_DFHACK_LIB
     template<class Enum, class FT> struct identity_traits<enum_field<Enum,FT> > {
-        static primitive_identity *get();
+        static enum_identity *get();
     };
 #endif
 
@@ -631,8 +631,9 @@ namespace df
 
 #ifdef BUILD_DFHACK_LIB
     template<class Enum, class FT>
-    inline primitive_identity *identity_traits<enum_field<Enum,FT> >::get() {
-        return identity_traits<FT>::get();
+    inline enum_identity *identity_traits<enum_field<Enum,FT> >::get() {
+        static enum_identity identity(identity_traits<Enum>::get(), identity_traits<FT>::get());
+        return &identity;
     }
 #endif
 
diff --git a/library/include/LuaWrapper.h b/library/include/LuaWrapper.h
index b23e1f912..c5daed0fc 100644
--- a/library/include/LuaWrapper.h
+++ b/library/include/LuaWrapper.h
@@ -127,6 +127,9 @@ namespace LuaWrapper {
     struct DFRefHeader {
         void *ptr;
         const struct_field_info *field_info;
+        const void *tag_ptr;
+        const type_identity *tag_identity;
+        const char *tag_attr;
     };
 
     /**
diff --git a/plugins/devel/check-structures-sanity/check-structures-sanity.h b/plugins/devel/check-structures-sanity/check-structures-sanity.h
index da790edf0..b606e9d86 100644
--- a/plugins/devel/check-structures-sanity/check-structures-sanity.h
+++ b/plugins/devel/check-structures-sanity/check-structures-sanity.h
@@ -131,7 +131,7 @@ private:
     void dispatch_bitfield(const QueueItem &, const CheckedStructure &);
     void dispatch_enum(const QueueItem &, const CheckedStructure &);
     void dispatch_struct(const QueueItem &, const CheckedStructure &);
-    void dispatch_field(const QueueItem &, const CheckedStructure &, const struct_field_info *, const struct_field_info *);
+    void dispatch_field(const QueueItem &, const CheckedStructure &, struct_identity *, const struct_field_info *);
     void dispatch_class(const QueueItem &, const CheckedStructure &);
     void dispatch_buffer(const QueueItem &, const CheckedStructure &);
     void dispatch_stl_ptr_vector(const QueueItem &, const CheckedStructure &);
diff --git a/plugins/devel/check-structures-sanity/dispatch.cpp b/plugins/devel/check-structures-sanity/dispatch.cpp
index f6342d03c..cd288ebfe 100644
--- a/plugins/devel/check-structures-sanity/dispatch.cpp
+++ b/plugins/devel/check-structures-sanity/dispatch.cpp
@@ -528,11 +528,11 @@ void Checker::dispatch_struct(const QueueItem & item, const CheckedStructure & c
 
         for (auto field = fields; field->mode != struct_field_info::END; field++)
         {
-            dispatch_field(item, cs, fields, field);
+            dispatch_field(item, cs, identity, field);
         }
     }
 }
-void Checker::dispatch_field(const QueueItem & item, const CheckedStructure & cs, const struct_field_info *fields, const struct_field_info *field)
+void Checker::dispatch_field(const QueueItem & item, const CheckedStructure & cs, struct_identity *identity, const struct_field_info *field)
 {
     if (field->mode == struct_field_info::OBJ_METHOD ||
         field->mode == struct_field_info::CLASS_METHOD)
@@ -544,7 +544,7 @@ void Checker::dispatch_field(const QueueItem & item, const CheckedStructure & cs
     QueueItem field_item(item, field->name, field_ptr);
     CheckedStructure field_cs(field);
 
-    auto tag_field = find_union_tag(fields, field);
+    auto tag_field = find_union_tag(identity, field);
     if (tag_field)
     {
         auto tag_ptr = PTR_ADD(item.ptr, tag_field->offset);
diff --git a/test/structures/unions.lua b/test/structures/unions.lua
index be469da6f..714786a96 100644
--- a/test/structures/unions.lua
+++ b/test/structures/unions.lua
@@ -15,6 +15,7 @@ function test.unit_action_type()
             expect.true_(name, "unit_action_type entry without name: " .. tostring(index))
             local tag = df.unit_action_type.attrs[name].tag
             expect.true_(tag, "unit_action_type entry missing tag: name=" .. name)
+            action.type = index
             expect.pairs_contains(action.data, tag,
                 "unit_action_type entry missing from unit_action.data: name=" .. name)
         end