This report was published in F. L. Hamer, W. Brauer and H. Schwichtenberg, editors, Logic and Algebra of Specification. Springer-Verlag, 1993.

The pi-calculus is a model of concurrent computation based upon the notion of naming. It is first presented in its simplest and original form, with the help of several illustrative applications. Then it is generalized from monadic to polyadic form. Semantics is done in terms of both a reduction system and a version of labelled transitions called commitment; the known algebraic axiomatization of strong bisimilarity is given in the new setting, and so also is a characterization in modal logic. Some theorems about the replication operator are proved. Justification for the polyadic form is provided by the concept of sort, sorting and sort discipline which it supports. Several illustrations of different sortings are given. One example is the presentation of data structures as processes which respect a particular sorting; another is the sorting for a known translation of the lambda-calculus in to pi-calculus. For this translation, the equational validity of beta-conversion is proved with the help of replication theorems. The paper ends with an extension of the pi-calculus to w-order processes, and a brief account of the demonstration by Davide Sangiorgi that higher-order processes may be faithfully encoded at first-order. This extends and strengthens the original result of this kind given by Bent Thomsen for second-order processes.