This book covers the proceedings of a meeting of the Association for Symbolic Logic held in Helsinki, Finland, in July 1990. It contains 18 papers written by leading researchers in logic, including most of the invited speakers of the meeting. They cover all fields of mathematical logic from the philosophy of mathematics through model theory, proof theory, recursion theory and set theory, to connections of logic to computer science. The articles published here are still widely referred to and continue to provide ideas for ongoing research projects.