Formal models of parallel computations are widely used in the design of parallel and distributed software (PDS) for multiprocessor computer systems (MCS). The choice of a model is decisive for efficiency of the solution of design problems. The necessity of such a choice is due to a splash of interest to high-level models of parallel processes. However, not all of them completely fit the problems of PDS design. The apparatus of PS-nets is proposed. On the one hand, it contains high-level means to describe interaction between and properties of the parallel processes. On the other hand, it is substantiated with a rigorous mathematical theory. Various stages of PDS design with the use of PS-nets are considered in the paper.