This book constitutes the strictly refereed post-workshop proceedings of the 12th International Workshop on Computer Science Logic, CSL '98, held as the Annual Conference of the European Association on Computer Science Logic in Brno, Czech Republic in August 1998. The 25 revised full papers presented were carefully reviewed and selected during two rounds of reviewing and revision. Also included are three reviewed invited papers. The papers span the whole scope of computer science logic and mathematical foundations and represent the state of the art in the area.