This book constitutes the thoroughly refereed post-proceedings of the 13th International Workshop on the Implementation of Functional Languages, IFL 2001, held in Stockholm, Sweden in September 2001. The eleven revised full papers presented have gone through a thorough round of post-workshop reviewing and were selected from 28 workshop papers. Among the topics covered are relevant aspects of implementing and using functional languages, such as type systems, compilation, program optimization, theorem proving, program correctness, program analysis, parallel compilers, subtyping, and generic programming.