CCS for OO and LP -- An extended expansion theorem -- Concurrent abstract machines -- Knowledge and probability in distributed systems: Abstract -- Verification methods for finite systems -- Interactive interworking for interoperating systems -- Formal specification of object systems -- On the relationship between algebraic module specifications and program modules -- Testing against formal specifications: A theoretical view -- Construction and reuse of formal program developments -- A theory of program modifications -- Proving termination of logic programs by exploiting term properties -- Parametric order-sorted types in logic programming -- Exploiting non-determinism through laziness in guarded functional languages -- Non-standard interpretations of LOTOS specifications -- A new technique for strictness analysis -- Using higher order logic for modelling real-time protocols -- Combining interaction and automation in process algebra verification -- Refining interfaces of communicating systems -- Structuring specifications in-the-large and in-the-small: Higher-order functions, dependent types and inheritance in SPECTRAL -- Actor-oriented system specification with dynamic logic -- Towards a formally based component description language -- On addition schemes -- Efficient code motion and an adaption to strength reduction -- On narrowing strategies for partial non-strict functions -- From reduction machines to narrowing machines -- The evaluation transformer model of reduction and its correctness.

TAPSOFT '91 is the Fourth International Joint Conference on Theory and Practice of Software Development. It was held in Brighton, April 8-12, 1991, and was organized by the Department of Computing, Imperial College, London. The proceedings of TAPSOFT '91 are organized into three parts: - Advances in Distributed Computing (ADC) - Colloquium on Trees in Algebra and Programming (CAAP) - Colloquium on Combining Paradigms for Software Development (CCPSD) The proceedings are published in two volumes. The first volume (LNCS, Vol. 493) contains the papers from CAAP. The second volume (LNCS, Vol. 494) contains the papers from the ADC and CCPSD. The ADC talks by distinguished invited speakers surveys current developments in distributed computing, including the integration of different paradigms for concurrency, algebraic, logical and operational foundations, and applications to software engineering and formal methods. The CCPSD papers address aspects of the trend in software enginering towards unification and synthesis combining theory and practice, and merging hitherto diverse approaches.