Methods and apparatus for dynamic class reloading and versioning that allow developers to change and recompile classes and to have running programs adopt the new versions of the classes dynamically, without redeploying the application. A dynamic class reloading component detects if the environment supports dynamic class redefinition and uses it if supported but does not require it. As the component loads a managed class, it modifies the bytecode of the class and generates additional classes and interfaces to support type-safe class versioning. Unique names are generated for successive versions of a managed class. A separate interface may be generated for each distinct method name and signature implemented on managed classes. Each generated class may implement all the generated interfaces that correspond to its methods. The same class loader that would load each managed class without the component loads the component-generated classes and interfaces.