In mental health care, psychologists assess clients' complaints, analyze underlying problems, and identify causes for these problems, to make treatment decisions. We present a study on psychologists' diagnostic processes, in which a mixed-method approach was employed. We aimed to identify a common structure in the diagnostic processes of different psychologists. We engaged an actor to simulate a client. Participants were asked to perform a diagnostic interview with this "client". This interview was videotaped. Afterwards participants first wrote a report and then were asked to review their considerations during the interview. We found that psychologists were comprehensive in their diagnostic interviews. They addressed the client's complaints, possible classifications, explanations, and treatments. They agreed about the classifications, more than about causal factors and treatment options. The content of the considerations differed between the interviews and the reports written afterwards. We conclude that psychologists continuously shifted between diagnostic activities and revised their decisions in line with the dynamics of the interview situation. Keywords: Clinical Decision Making, Diagnostic Interview, Stimulated Recall, Simulated Patient IntroductionPsychological assessment is a diagnostic decision making process aimed at describing, classifying, explaining, predicting, and often also changing the behavior of a client (Fernández-Ballesteros, 1999). The result of the diagnostic process is an integrated representation of a client's complaints and problems together with an explanation for the problems and a treatment proposal (Nelson-Gray, 2003). Guidelines have been developed to support psychologists in structured gathering and interpreting diagnostic information (see e.g. Groth-Marnat, 2003;Nezu & Nezu, 1995) to make sure that a thorough assessment is carried out and correct diagnostic decisions are made to reach the most adequate treatment proposal. Unfortunately, as Garb (1998) concluded, reliability and validity of these diagnostic decisions are low, although the final treatment proposals are often adequate. An explanation is provided in studies showing that psychologists do not always carry out all parts of the diagnostic process appropriately, even when explicitly asked to do so (Eells, Kendjelic, & Lucas, 1998;Groenier, Pieters, Hulshof, Wilhelm, & Witteman, 2008). In most of these studies assessment situations were artificial, and it remains unclear whether psychologists in actual practice would perform and report all parts of the diagnostic process. The current study is aimed at identifying common structures in psychologists' diagnostic information gathering processes in an authentic assessment situation.The diagnostic process involves using different instruments, such as tests, interviews, and observations, as well as performing different diagnostic activities, such as diagnostic classification and diagnostic formulation (Fernández-Ballesteros et al., 2001). Diagnostic classification and diagnostic ...