This paper introduces higher-order processes as a means to describe complex human-computer interaction systems by integrating different HCI theories. A higher-order process consists of components, which are (sub-)processes themselves. The behaviour of a process results from the behaviour of its components and their interaction. While processes at the lowest level describe the essential units of a system, higher-level processes focus on the way their components interact and produce a new kind of behaviour. Such a behaviour can roughly be characterized as ?more planned?. We further propose a convenient specification formalism for higher-order processes based on partial equations.