Process theory has become an important mechanism for the accumulation of knowledge in a number of disciplines. In contrast with variance theory, which focuses on co-variation of dependent and independent variables, process theory focuses on sequences of activities, their duration and the intervals between them, as they lead to particular outcomes. For example, rather than focusing on what properties successful projects have in common, process theory focuses on what sequences of activities lead to successful projects. Thus process theory is a powerful companion to variance theory, particularly relevant to project management knowledge accumulation. However, process theory itself and methods of developing process theories are still in a nascent stage. We present a 5-level model that specifies different formulations of process theory, explaining how it can be applied and leveraged to accumulate knowledge, specifically within project management research. We conclude by considering future possibilities and challenges for process theory in project management research.