The book "Semantics and Algebraic Specification" is a festschrift published in honor of Peter D. Mosses on the occasion of his 60th birthday. It includes 17 inviting chapters written by many of Peter D. Mosses' co-authors, collaborators, close colleagues, and former students. Peter D. Mosses is known for his significant contributions in the field of formal programming semantics, particularly for the development of action semantics, which represents a combination of denotational, operational, and algebraic semantics. The presentations gathered in this volume stem from a symposium held in his honor. The topics covered are diverse and reflect Peter's technical contributions, including action semantics, security policy design, colored Petri nets, object-oriented action semantics, and structural operational semantics. This festschrift honors his lasting influence on the field and provides valuable insights into current research topics.