The innovative progress in the development oflarge-and small-scale parallel computing systems and their increasing availability have caused a sharp rise in interest in the scientific principles that underlie parallel computation and parallel programming. The biannual "Parallel Architectures and Languages Europe" (PARLE) conferences aim at presenting current research material on all aspects of the theory, design, and application of parallel computing systems and parallel processing. At the same time, the goal of the PARLE conferences is to provide a forum for researchers and practitioners to ex­ change ideas on recent developments and trends in the field of parallel com­ puting and parallel programming. The first ~wo conferences, PARLE '87 and PARLE '89, have succeeded in meeting this goal and made PARLE a conference that is recognized worldwide in the field of parallel computation. PARLE '91 again offers a wealth of high-quality research material for the benefit of the scientific community. Compared to its predecessors, the scope of PARLE '91 has been broadened so as to cover the area of parallel algo­ rithms and complexity, in addition to the central themes of parallel archi­ tectures and languages. The proceedings of the PARLE '91 conference contain the text of all con­ tributed papers that were selected for the programme and of the invited papers by leading experts in the field.