. We present a general theory of behavioural subtyping for name passing interactive behaviours using early name-passing synchronisation trees. In this theory types are collections of name passing synchronisation trees organised by typed variants of processtheoretic operations, and a simple behavioural notion of subtyping specifies when one type denotes more constrained behaviours than another, offering a semantic basis for diverse instances of subtyping in sequential and concurrent computation through their representation in name passing. The robustness of the notion is shown by a few equivalent characterisations, including the one based on the subset inclusion with respect to inhabitants of types and another concerning a basic substitutability property. As an application, we show how the subtyping in the -calculus with constant data domains is soundly embeddable into the present theory, illuminating the functional notion of subtyping from a behavioural viewpoint. 1. Introduction The ...