diff --git a/src/prefixer/File.ts b/src/prefixer/File.ts
index c519189..aadac0a 100644
--- a/src/prefixer/File.ts
+++ b/src/prefixer/File.ts
@@ -6,8 +6,8 @@ import { buildAst } from '@xml-tools/ast';
 import type { RopmOptions } from '../util';
 import { util } from '../util';
 import * as path from 'path';
-import type { BrsFile, Position, Program, XmlFile } from 'brighterscript';
-import { createVisitor, isCallExpression, isDottedGetExpression, isDottedSetStatement, isIndexedGetExpression, isIndexedSetStatement, WalkMode } from 'brighterscript';
+import type { BrsFile, Position, Program, Range, XmlFile } from 'brighterscript';
+import { ParseMode, createVisitor, isCallExpression, isCustomType, isDottedGetExpression, isDottedSetStatement, isIndexedGetExpression, isIndexedSetStatement, WalkMode, util as bsUtil } from 'brighterscript';
 
 export class File {
     constructor(
@@ -94,6 +94,15 @@ export class File {
         endOffset: number;
     }>;
 
+    /**
+     * Anywhere that a class is used as a type (like in class `extends` or function parameters)
+     */
+    public classReferences = [] as Array<{
+        fullyQualifiedName: string;
+        offsetBegin: number;
+        offsetEnd: number;
+    }>;
+
     public functionReferences = [] as Array<{
         name: string;
         offset: number;
@@ -231,12 +240,31 @@ export class File {
         }
     }
 
+    private addClassRef(className: string, containingNamespace: string | undefined, range: Range) {
+        //look up the class. If we can find it, use it
+        const cls = (this.bscFile as BrsFile).getClassFileLink(className, containingNamespace)?.item;
+
+        let fullyQualifiedName: string;
+        if (cls) {
+            fullyQualifiedName = bsUtil.getFullyQualifiedClassName(cls.getName(ParseMode.BrighterScript), cls.namespaceName?.getName(ParseMode.BrighterScript));
+        } else {
+            fullyQualifiedName = bsUtil.getFullyQualifiedClassName(className, containingNamespace);
+        }
+
+        this.classReferences.push({
+            fullyQualifiedName: fullyQualifiedName,
+            offsetBegin: this.positionToOffset(range.start),
+            offsetEnd: this.positionToOffset(range.end)
+        });
+    }
+
     /**
      * find various items from this file.
      */
     public walkAst() {
+        const file = this.bscFile as BrsFile;
         /* eslint-disable @typescript-eslint/naming-convention */
-        (this.bscFile as BrsFile).parser.ast.walk(createVisitor({
+        file.parser.ast.walk(createVisitor({
             ImportStatement: (stmt) => {
                 //skip pkg paths, those are collected elsewhere
                 if (!stmt.filePath.startsWith('pkg:/')) {
@@ -283,6 +311,34 @@ export class File {
                     ),
                     endOffset: this.positionToOffset(cls.end.range.end)
                 });
+
+                if (cls.parentClassName) {
+                    this.addClassRef(
+                        cls.parentClassName.getName(ParseMode.BrighterScript),
+                        cls.namespaceName?.getName(ParseMode.BrighterScript),
+                        cls.parentClassName.range
+                    );
+                }
+            },
+            FunctionExpression: (func) => {
+                const namespaceName = func.namespaceName?.getName(ParseMode.BrighterScript);
+                //any parameters containing custom types
+                for (const param of func.parameters) {
+                    if (isCustomType(param.type)) {
+                        this.addClassRef(
+                            param.type.name,
+                            namespaceName,
+                            param.typeToken!.range
+                        );
+                    }
+                }
+                if (isCustomType(func.returnType)) {
+                    this.addClassRef(
+                        func.returnType.name,
+                        namespaceName,
+                        func.returnTypeToken!.range
+                    );
+                }
             },
             FunctionStatement: (func) => {
                 this.functionDefinitions.push({
diff --git a/src/prefixer/ModuleManager.spec.ts b/src/prefixer/ModuleManager.spec.ts
index aa75ac8..1c42833 100644
--- a/src/prefixer/ModuleManager.spec.ts
+++ b/src/prefixer/ModuleManager.spec.ts
@@ -1272,6 +1272,165 @@ describe('ModuleManager', () => {
             });
         });
 
+        it('prefixes classes used in parameters', async () => {
+            await testProcess({
+                'logger:source/lib.d.bs': [
+                    trim`
+                        namespace animals
+                            class Dog
+                                sub new(brother as Dog, sister as animals.Dog, owner as Human)
+                                end sub
+                            end class
+                        end namespace
+
+                        class Human
+                        end class
+                    `,
+                    trim`
+                        namespace logger.animals
+                            class Dog
+                                sub new(brother as logger.animals.Dog, sister as logger.animals.Dog, owner as logger.Human)
+                                end sub
+                            end class
+                        end namespace
+
+                        namespace logger
+                        class Human
+                        end class
+                        end namespace
+                    `
+                ]
+            });
+        });
+
+        it('prefixes classes used in extends', async () => {
+            await testProcess({
+                'logger:source/lib.d.bs': [
+                    trim`
+                        namespace animals
+                            class Animal
+                            end class
+                            class Dog extends Animal
+                            end class
+                            class Cat extends animals.Animal
+                            end class
+                            class Warewolf extends Human
+                            end class
+                        end namespace
+                        class Human
+                        end class
+                        class Warecat extends animals.Cat
+                        end class
+                    `,
+                    trim`
+                        namespace logger.animals
+                            class Animal
+                            end class
+                            class Dog extends logger.animals.Animal
+                            end class
+                            class Cat extends logger.animals.Animal
+                            end class
+                            class Warewolf extends logger.Human
+                            end class
+                        end namespace
+                        namespace logger
+                        class Human
+                        end class
+                        end namespace
+                        namespace logger
+                        class Warecat extends logger.animals.Cat
+                        end class
+                        end namespace
+                    `
+                ]
+            });
+        });
+
+        it('prefixes classes used as return type', async () => {
+            await testProcess({
+                'logger:source/lib.d.bs': [
+                    trim`
+                        namespace animals
+                            class Dog
+                            end class
+                            function GetDog1() as Dog
+                            end function
+                            function GetDog2() as animals.Dog
+                            end function
+                            function GetHuman() as Human
+                            end function
+                        end namespace
+
+                        function GetDog3() as animals.Dog
+                        end function
+
+                        class Human
+                        end class
+                    `,
+                    trim`
+                        namespace logger.animals
+                            class Dog
+                            end class
+                            function GetDog1() as logger.animals.Dog
+                            end function
+                            function GetDog2() as logger.animals.Dog
+                            end function
+                            function GetHuman() as logger.Human
+                            end function
+                        end namespace
+
+                        namespace logger
+                        function GetDog3() as logger.animals.Dog
+                        end function
+                        end namespace
+
+                        namespace logger
+                        class Human
+                        end class
+                        end namespace
+                    `
+                ]
+            });
+        });
+
+        it('does not prefix other module namespaced class names', async () => {
+            manager.modules = createProjects(hostDir, hostDir, {
+                name: 'host',
+                dependencies: [{
+                    name: 'logger',
+                    _files: {
+                        'source/lib.d.bs': trim`
+                            class Person
+                                sub new(pet as a.Duck)
+                                end sub
+                                sub watchPetForFriend(friendPet as dogs.Poodle)
+                                end sub
+                            end class
+                        `
+                    },
+                    dependencies: [{
+                        name: 'animals',
+                        alias: 'a'
+                    }, {
+                        name: 'dogs'
+                    }]
+                }]
+            });
+
+            await process();
+
+            fsEqual(`${hostDir}/source/roku_modules/logger/lib.d.bs`, `
+                namespace logger
+                class Person
+                    sub new(pet as animals_v1.Duck)
+                    end sub
+                    sub watchPetForFriend(friendPet as dogs_v1.Poodle)
+                    end sub
+                end class
+                end namespace
+            `);
+        });
+
         it('properly handles annotations', async () => {
             await testProcess({
                 'logger:source/lib.d.bs': [
diff --git a/src/prefixer/RopmModule.ts b/src/prefixer/RopmModule.ts
index 7f3fb8f..bf8ead6 100644
--- a/src/prefixer/RopmModule.ts
+++ b/src/prefixer/RopmModule.ts
@@ -311,6 +311,21 @@ export class RopmModule {
         const ownComponentNames = this.getDistinctComponentDeclarationNames();
         const prefixMapKeys = Object.keys(this.prefixMap);
         const prefixMapKeysLower = prefixMapKeys.map(x => x.toLowerCase());
+
+        /**
+         * Get the alias for a namespace. Only returns if it exists and is different than what is given.
+         */
+        const getAlias = (namespace?: string) => {
+            if (namespace) {
+                const lowerNamespaceName = namespace.toLowerCase();
+                const idx = prefixMapKeysLower.indexOf(lowerNamespaceName);
+                const prefix = this.prefixMap[prefixMapKeys[idx]];
+                if (prefix && prefix.toLowerCase() !== lowerNamespaceName) {
+                    return prefix;
+                }
+            }
+        };
+
         const nonPrefixedFunctionMap = {
             ...this.nonPrefixedFunctionMap,
             ...this.getInterfaceFunctions()
@@ -362,6 +377,22 @@ export class RopmModule {
                     }
                 }
 
+                //prefix d.bs class references
+                for (const ref of file.classReferences) {
+                    const baseNamespace = util.getBaseNamespace(ref.fullyQualifiedName);
+
+                    const alias = getAlias(baseNamespace);
+                    let fullyQualifiedName: string;
+                    //if we have an alias, this is a class from another module.
+                    if (alias) {
+                        fullyQualifiedName = ref.fullyQualifiedName.replace(/^.*?\./, alias + '.');
+                    } else {
+                        //this is an internal-module class, so append our prefix to it
+                        fullyQualifiedName = `${brighterscriptPrefix}.${ref.fullyQualifiedName}`;
+                    }
+                    file.addEdit(ref.offsetBegin, ref.offsetEnd, fullyQualifiedName);
+                }
+
                 //prefix d.bs namespaces
                 for (const namespace of file.namespaces) {
                     file.addEdit(namespace.offset, namespace.offset, brighterscriptPrefix + '.');
diff --git a/src/util.ts b/src/util.ts
index 5ad76c0..129ac1d 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -283,6 +283,16 @@ export class Util {
         }
     }
 
+    /**
+     * Get the base namespace from a namespace statement, or undefined if there are no dots
+     */
+    public getBaseNamespace(text: string) {
+        const parts = text.split('.');
+        if (parts.length > 1) {
+            return parts[0];
+        }
+    }
+
 }
 
 function mockProgramValidate() {