The innovative progress in the development of large- 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 on all aspects of the theory, design and application of parallel computing systems and parallel processing. PARLE '91, the third conference in the series, 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 algorithms and complexity, in addition to the central themes of parallel architectures and languages. The two-volume proceedings of the PARLE '91 conference contain the text of all contributed papers that were selected for the programme and of the invited papers by leading experts in the field.