This Festschrift volume, published in honor of Symeon Bozapalidis on the occasion of his retirement after more than 35 years of teaching activity, focuses on the subjects taught by Symeon, namely: algebra, linear algebra, mathematical logic, number theory, automata theory, tree languages and series, algebraic semantics, and fuzzy languages. Since 1982 -- at the Aristotle University of Thessaloniki -- Symeon's main interests have been closely connected with the algebraic foundations in computer science. In particular, he contributed to the development of the theory of tree languages and series, the axiomatization of graphs, picture theory, and fuzzy languages.The volume contains 15 invited papers, written by colleagues, friends, and students of Symeon. All of the papers were carefully refereed and are connected to his research topics. Most of the papers were presented at the Workshop on Algebraic Foundations in Computer Science, held in Thessaloniki, Greece, during November 7--8, 2011.