Psychological formulations are central to cognitive behavioural approaches. The use of such formulations presents a number of difficulties when working with clients with psychotic problems. Despite this, sophisticated psychological formulations can be collaboratively developed with psychotic clients. This paper presents one method of developing such formulations through an evolutionary process. Early in the therapeutic process, simple formulations involving straightforward theoretical models are presented, which are systematically elaborated as therapy proceeds. This involves developing, collaboratively with clients, successive layers of formulation. Each of these layers builds on and incorporates the previous one, yet involves an incremental increase in complexity, depth and informational content. The evolutionary process is illustrated with a case example.