This book constitutes the thoroughly refereed post-conference proceedings of the 12th International Symposium on Trends in Functional Programming, TFP 2011, held in Madrid, Spain, in May 2011. The 12 papers presented were carefully reviewed and selected from 21 submissions. They deal with all aspects of functional programming, taking a broad view of current and future trends in this area. The topical sections the papers are organized in are named as follows: types, compiling, paralelelism and distribution, data structures, and miscellaneous.