One of the most significant limitations of the Java Programming Language is its lack of support for generic types. This thesis describes an efficient compiler and run-time type system for NextGen, a compatible extension of Java that supports generic types, including type dependent operations. The NextGen compiler is implemented as an extension to the existing compiler for GJ, a limited generic extension of Java that does not support run-time generic types. Our implementation is homogeneous, with one major exception: Instantiations of generic classes are constructed on demand. This exception is necessary to support polymorphic recursion. The NextGen compiler generates templates for such instantiation classes and relies on a customized class loader to construct instantiations at run-time. A suite of benchmarks on the Java, GJ, and NextGen compilers shows that the additional code required to support run-time generic types has little or no overhead compared to ordinary Java and GJ.