Abstract. Theories defined in a process model are formalized and studied. A theory in a process calculus is a set of perpetually available processes with finite interactability, each can be regarded as a service, an agent behind the scene or an axiom. The operational and observational semantics of the theories are investigated. The power of the approach is demonstrated by interpreting the asynchronous -calculus as a theory, the asynchronous theory, in the -calculus. A complete axiomatic system is constructed for the asynchronous theory, which gives rise to a proof system for the weak asynchronous bisimilarity of the asynchronous .