This book constitutes the refereed proceedings of the Second International Workshop on Human Interactive Proofs, HIP 2005, held in Bethlehem, PA, USA in May 2005.

The 9 revised full papers presented were carefully reviewed and selected for presentation. This book is the first archival publication devoted to the new class of security protocols called human interactive proofs. The variant of HIPs being used in e-commerce and Web services today is completely automatic public Turing tests to tell computers and humans apart, CAPTCHAs. The papers in this book are organized in topical sections on CAPTCHAs and performance analysis, HIP architectures, HIP as security systems components.