TAPSOFT '89 is the Third International Joint Conference on Theory and Practice of Software Development held in Barcelona, Spain, March 13-17, 1989. The conference consisted of three parts: - Advanced Seminar on Foundations of Innovative Software Development - Colloquium on Trees in Algebra and Programming (CAAP '89) - Colloquium on Current Issues in Programming Languages (CCIPL) The TAPSOFT '89 Conference Proceedings are published in two volumes. The first volume includes the papers from CAAP plus the more theoretical ones of the invited papers. The second volume comprises the papers from CCIPL and the invited papers more relevant to current issues in programming languages.