The Ninth ACM SIGPLAN International Conference on Functional Programming (ICFP) took place on September 19–21 2004 in Snowbird, Utah. The scope of ICFP includes all languages that encourage programming with functions, with topics ranging from principles to practice, foundations to features, and abstractions to applications. The program committee, which I chaired, selected 21 papers from 80 submissions for presentation at the conference. Afterwards, the program committee invited the authors of nine papers to submit extended versions for this special issue of JFP. The seven papers that appear in this volume were reviewed, revised, and accepted following standard Journal of Functional Programming procedures. Reflecting the scope of the conference, the papers cover the theoretical foundations, implementation, design, programming techniques, and applications of functional languages.