This book constitutes the refereed proceedings of the 6th International Symposium on Abstraction, Reformulation, and Approximation, SARA 2005, held in Airth Castle, Scotland, UK in July 2005. The 17 revised full papers and 8 extended abstracts were carefully reviewed and selected for inclusion in the book. Also included are 3 invited papers and 8 research summaries. All current aspects of abstraction, reformulation, and approximation in the context of human common-sense reasoning, problem solving, and efficiently reasoning in complex domains are addressed. Among the application fields of these techniques are automatic programming, constraint satisfaction, design, diagnosis, machine learning, search, planning, reasoning, game playing, scheduling, and theorem proving.