The present volume contains most of the papers presented at the Symposium A 'Defects in Silicon: Hydrogen'. The symposium took place at the 1998 Spring Meeting of the European Materials Research Society from June 16-19 1998, Strasbourg, France, and was devoted to the fundamental aspects of hydrogen in crystalline silicon and their influence on silicon technology. The two-and-a-half day scientific program included 13 invited and 35 contributed papers.