This book constitutes the thoroughly refereed post-proceedings of the 12th International Workshop on Implementing Functional Languages, IFL 2000, held in Aachen, Germany in September 2000.The 15 revised full papers presented have gone through a thorough round of post-workshop reviewing and were selected from 33 workshop presentations. Among the topics covered are language concepts, type checking, compilation techniques, abstract interpretation, automatic program generation, abstract machine architectures, array processing, concurrent and parallel processing, heap management, runtime profiling, performance measurement, debugging and tracing, and tools and programming techniques.