This book contains a strictly refereed selection of revised full papers chosen from the papers accepted for presentation during the 11th Workshop on Abstract Data Types held jointly with the 8th COMPASS Workshop in Oslo, Norway, in September 1995.The 25 research papers included were chosen from 57 pre-selected workshop presentations; also included are six invited contributions. The volume reports the progress achieved in the area of algebraic specification since the predecessor meeting held in May 1994.