This Festschrift has been published in honor of Rinus Plasmeijer, to celebrate the combined occasion of his 61st birthday and the 25th Symposium on Implementation and Application of Functional Languages, IFL 2013, held in Nijmegen, The Netherlands, in August 2013. Rinus Plasmeijer was the main designer of the lazy functional programming language "Clean" and has always been the leader of the associated research team. He has played a decisive role in making the Radboud University of Nijmegen an important center of research in functional programming by organizing and hosting the first few IFL symposia in Nijmegen. This Festschrift contains 19 scientific essays written by former PhD students of Rinus Plasmeijer and researchers in the field of functional programming who have collaborated with him. The authors write about the influence the beauty of functional programming has had or still has on their work.

Beautiful Code, Beautiful Proof?.- Beauty and Code.- An Ontology of States.- Functional Type Assignment for Featherweight Java: (To Rinus Plasmeijer, in Honour of His 61st Birthday).- Verifying Functional Formalizations - A Type-Theoretical Case Study in PVS.- Functional Semantics.- Why Functional Programming Matters to Me.- Clocks for Functional Programs.- Declarative Natural Language Specifications.- Clean Up the Web!: (Rapid Client-Side Web Development with Clean).- A Duality of Sorts.- Programming in the ¿-Calculus: From Church to Scott and Back.- Modelling Unique and Affine Typing Using Polymorphism.- Evolution of a Parallel Task Combinator.- Beautiful Workflows: A Matter of Taste?.- Parse Your Options.- The Beauty of Declarative Purely Functional Projections.- Finding Palindromes: Variants and Algorithms.- Beautiful Imperative Code: A Functional Ace in the Hole for Imperative Programmers.