diff --git a/CesiumGltf/include/CesiumGltf/Model.h b/CesiumGltf/include/CesiumGltf/Model.h
index fd379f389..e8a4642fb 100644
--- a/CesiumGltf/include/CesiumGltf/Model.h
+++ b/CesiumGltf/include/CesiumGltf/Model.h
@@ -1,7 +1,8 @@
 #pragma once
 
-#include "CesiumGltf/Library.h"
-#include "CesiumGltf/ModelSpec.h"
+#include <CesiumGltf/Library.h>
+#include <CesiumGltf/ModelSpec.h>
+#include <CesiumUtility/ErrorList.h>
 
 #include <glm/mat4x4.hpp>
 
@@ -22,7 +23,7 @@ struct CESIUMGLTF_API Model : public ModelSpec {
    *
    * @param rhs The model to merge into this one.
    */
-  void merge(Model&& rhs);
+  CesiumUtility::ErrorList merge(Model&& rhs);
 
   /**
    * @brief A callback function for {@link forEachPrimitiveInScene}.
diff --git a/CesiumGltf/src/Model.cpp b/CesiumGltf/src/Model.cpp
index 42ee27c3b..ef138804a 100644
--- a/CesiumGltf/src/Model.cpp
+++ b/CesiumGltf/src/Model.cpp
@@ -1,7 +1,10 @@
 #include "CesiumGltf/Model.h"
 
 #include "CesiumGltf/AccessorView.h"
+#include "CesiumGltf/ExtensionExtMeshFeatures.h"
 #include "CesiumGltf/ExtensionKhrDracoMeshCompression.h"
+#include "CesiumGltf/ExtensionMeshPrimitiveExtStructuralMetadata.h"
+#include "CesiumGltf/ExtensionModelExtStructuralMetadata.h"
 
 #include <glm/gtc/quaternion.hpp>
 #include <glm/gtx/norm.hpp>
@@ -9,9 +12,14 @@
 #include <gsl/span>
 
 #include <algorithm>
+#include <charconv>
+
+using namespace CesiumUtility;
 
 namespace CesiumGltf {
+
 namespace {
+
 template <typename T>
 size_t copyElements(std::vector<T>& to, std::vector<T>& from) {
   const size_t out = to.size();
@@ -29,9 +37,24 @@ void updateIndex(int32_t& index, size_t offset) noexcept {
   }
   index += int32_t(offset);
 }
+
+void updateIndex(int64_t& index, size_t offset) noexcept {
+  if (index == -1) {
+    return;
+  }
+  index += int64_t(offset);
+}
+
+void mergeSchemas(
+    Schema& lhs,
+    Schema& rhs,
+    std::map<std::string, std::string>& classNameMap);
+
 } // namespace
 
-void Model::merge(Model&& rhs) {
+ErrorList Model::merge(Model&& rhs) {
+  ErrorList result;
+
   // TODO: we could generate this pretty easily if the glTF JSON schema made
   // it clear which index properties refer to which types of objects.
 
@@ -65,6 +88,83 @@ void Model::merge(Model&& rhs) {
   const size_t firstSkin = copyElements(this->skins, rhs.skins);
   const size_t firstTexture = copyElements(this->textures, rhs.textures);
 
+  size_t firstPropertyTable = 0;
+  size_t firstPropertyTexture = 0;
+  size_t firstPropertyAttribute = 0;
+
+  ExtensionModelExtStructuralMetadata* pRhsMetadata =
+      rhs.getExtension<ExtensionModelExtStructuralMetadata>();
+  if (pRhsMetadata) {
+    ExtensionModelExtStructuralMetadata& metadata =
+        this->addExtension<ExtensionModelExtStructuralMetadata>();
+
+    if (metadata.schemaUri && pRhsMetadata->schemaUri &&
+        *metadata.schemaUri != *pRhsMetadata->schemaUri) {
+      // We can't merge schema URIs. So the thing to do here is download both
+      // schemas and merge them. But for now we're just reporting an error.
+      result.emplaceError("Cannot merge EXT_structural_metadata extensions "
+                          "with different schemaUris.");
+    } else if (pRhsMetadata->schemaUri) {
+      metadata.schemaUri = pRhsMetadata->schemaUri;
+    }
+
+    std::map<std::string, std::string> classNameMap;
+
+    if (metadata.schema && pRhsMetadata->schema) {
+      mergeSchemas(*metadata.schema, *pRhsMetadata->schema, classNameMap);
+    } else if (pRhsMetadata->schema) {
+      metadata.schema = std::move(pRhsMetadata->schema);
+    }
+
+    firstPropertyTable =
+        copyElements(metadata.propertyTables, pRhsMetadata->propertyTables);
+    firstPropertyTexture =
+        copyElements(metadata.propertyTextures, pRhsMetadata->propertyTextures);
+    firstPropertyAttribute = copyElements(
+        metadata.propertyAttributes,
+        pRhsMetadata->propertyAttributes);
+
+    for (size_t i = firstPropertyTable; i < metadata.propertyTables.size();
+         ++i) {
+      PropertyTable& propertyTable = metadata.propertyTables[i];
+      auto it = classNameMap.find(propertyTable.classProperty);
+      if (it != classNameMap.end()) {
+        propertyTable.classProperty = it->second;
+      }
+
+      for (std::pair<const std::string, PropertyTableProperty>& pair :
+           propertyTable.properties) {
+        updateIndex(pair.second.values, firstBufferView);
+        updateIndex(pair.second.arrayOffsets, firstBufferView);
+        updateIndex(pair.second.stringOffsets, firstBufferView);
+      }
+    }
+
+    for (size_t i = firstPropertyTexture; i < metadata.propertyTextures.size();
+         ++i) {
+      PropertyTexture& propertyTexture = metadata.propertyTextures[i];
+      auto it = classNameMap.find(propertyTexture.classProperty);
+      if (it != classNameMap.end()) {
+        propertyTexture.classProperty = it->second;
+      }
+
+      for (std::pair<const std::string, PropertyTextureProperty>& pair :
+           propertyTexture.properties) {
+        updateIndex(pair.second.index, firstTexture);
+      }
+    }
+
+    for (size_t i = firstPropertyAttribute;
+         i < metadata.propertyAttributes.size();
+         ++i) {
+      PropertyAttribute& propertyAttribute = metadata.propertyAttributes[i];
+      auto it = classNameMap.find(propertyAttribute.classProperty);
+      if (it != classNameMap.end()) {
+        propertyAttribute.classProperty = it->second;
+      }
+    }
+  }
+
   // Update the copied indices
   for (size_t i = firstAccessor; i < this->accessors.size(); ++i) {
     Accessor& accessor = this->accessors[i];
@@ -122,6 +222,32 @@ void Model::merge(Model&& rhs) {
       if (pDraco) {
         updateIndex(pDraco->bufferView, firstBufferView);
       }
+
+      ExtensionMeshPrimitiveExtStructuralMetadata* pMetadata =
+          primitive.getExtension<ExtensionMeshPrimitiveExtStructuralMetadata>();
+      if (pMetadata) {
+        for (int64_t& propertyTextureID : pMetadata->propertyTextures) {
+          updateIndex(propertyTextureID, firstPropertyTexture);
+        }
+
+        for (int64_t& propertyAttributeID : pMetadata->propertyAttributes) {
+          updateIndex(propertyAttributeID, firstPropertyAttribute);
+        }
+      }
+
+      ExtensionExtMeshFeatures* pMeshFeatures =
+          primitive.getExtension<ExtensionExtMeshFeatures>();
+      if (pMeshFeatures) {
+        for (FeatureId& featureId : pMeshFeatures->featureIds) {
+          if (featureId.propertyTable) {
+            updateIndex(*featureId.propertyTable, firstPropertyTable);
+          }
+
+          if (featureId.texture) {
+            updateIndex(featureId.texture->index, firstTexture);
+          }
+        }
+      }
     }
   }
 
@@ -215,6 +341,8 @@ void Model::merge(Model&& rhs) {
 
     this->scene = int32_t(this->scenes.size() - 1);
   }
+
+  return result;
 }
 
 namespace {
@@ -694,4 +822,85 @@ bool Model::isExtensionRequired(
              extensionName) != this->extensionsRequired.end();
 }
 
+namespace {
+
+template <typename T>
+std::string findAvailableName(
+    const std::unordered_map<std::string, T>& map,
+    const std::string& name) {
+  auto it = map.find(name);
+  if (it == map.end())
+    return name;
+
+  // Name already exists in the map, so find a numbered name that doesn't.
+
+  uint64_t number = 1;
+  while (number < std::numeric_limits<uint64_t>::max()) {
+    std::string newName = fmt::format("{}_{}", name, number);
+    it = map.find(newName);
+    if (it == map.end()) {
+      return newName;
+    }
+
+    ++number;
+  }
+
+  // Realistically, this can't happen. It would mean we checked all 2^64
+  // possible names and none of them are available.
+  assert(false);
+  return name;
+}
+
+void mergeSchemas(
+    Schema& lhs,
+    Schema& rhs,
+    std::map<std::string, std::string>& classNameMap) {
+  if (!lhs.name)
+    lhs.name = rhs.name;
+  else if (rhs.name && *lhs.name != *rhs.name)
+    lhs.name.emplace("Merged");
+
+  if (!lhs.description)
+    lhs.description = rhs.description;
+  else if (rhs.description && *lhs.description != *rhs.description)
+    lhs.description.emplace("This is a merged schema created by combining "
+                            "together the schemas from multiple glTFs.");
+
+  if (!lhs.version)
+    lhs.version = rhs.version;
+  else if (rhs.version && *lhs.version != *rhs.version)
+    lhs.version.reset();
+
+  std::map<std::string, std::string> enumNameMap;
+
+  for (std::pair<const std::string, Enum>& pair : rhs.enums) {
+    std::string availableName = findAvailableName(lhs.enums, pair.first);
+    lhs.enums[availableName] = std::move(pair.second);
+    if (availableName != pair.first)
+      enumNameMap.emplace(pair.first, availableName);
+  }
+
+  for (std::pair<const std::string, Class>& pair : rhs.classes) {
+    std::string availableName = findAvailableName(lhs.classes, pair.first);
+    Class& klass = lhs.classes[availableName];
+    klass = std::move(pair.second);
+
+    if (availableName != pair.first)
+      classNameMap.emplace(pair.first, availableName);
+
+    // Remap enum names in class properties.
+    for (std::pair<const std::string, ClassProperty>& propertyPair :
+         klass.properties) {
+      if (propertyPair.second.enumType) {
+        auto it = enumNameMap.find(*propertyPair.second.enumType);
+        if (it != enumNameMap.end()) {
+          propertyPair.second.enumType = it->second;
+        }
+      }
+    }
+  }
+}
+
+} // namespace
+
 } // namespace CesiumGltf