The development of new and improved proof systems, proof formats and proof search methods is one of the most essential goals of Logic. But what is a proof? What makes a proof better than another? How can a proof be found efficiently? How can a proof be used? Logicians from different communities usually provide radically different answers to such questions. Their principles may be folklore within their own communities but are often unknown to outsiders. This book provides a snapshot of the current state of the art in proof search and proof production as implemented in contemporary automated reasoning tools such as SAT-solvers, SMT-solvers, first-order and higher-order automated theorem provers and proof assistants. Furthermore, various trends in proof theory, such as the calculus of inductive constructions, deduction modulo, deep inference, foundational proof certificates and cut-elimination, are surveyed; and applications of formal proofs are illustrated in the areas of cryptography, verification and mathematical proof mining. Experts in these topics were invited to present tutorials about proofs during the Vienna Summer of Logic and the chapters in this book reflect their tutorials. Therefore, each chapter is intended to be accessible not only to experts but also to novice researchers from all fields of Logic.