Process calculus is a family of related approaches to formally model concurrent systems, e.g. Pi-calculus which pass channels as data along other channels; Ambient calculus which models distributed and mobile computations. Rho Calculus (Reflective Higher Order Calculus) is based on Pi-calculus; it is a closed theory in the form of an asynchronous message-passing calculus built on a notion of quoting. Names are quoted processes and unquoting is reification of processes. Names are subject to algebraic rules and reasoning such as substitutions and bisimulation.