This book constitutes the thoroughly refereed post-proceedings of the Third International Workshop of the Types Working Group, TYPES 2003, held in Torino, Italy in April/May 2003.The 25 revised full papers presented were carefully selected during two rounds of reviewing and improvement. All current issues in type theory and type systems and their applications to programming, systems design, and proof theory are addressed. Among the systems dealt with are Isabelle/Isar, PAF!, and Coq.