In this article, we describe an investigation into the diagnostic activities of practicing clinical psychologists. Two questionnaires were filled in by 313 psychologists. One group of psychologists (N = 175) judged the necessity of diagnostic activities; the other group (N = 138) selected the activities they would actually perform. Results showed that more participants thought that diagnostic activities were necessary than there were participants who intended to actually perform those activities. Causal analysis, by generating and testing diagnostic hypotheses to form an integrated client model with an explanation for the problem, was judged least necessary and would not be performed. We conclude that a discrepancy exists between the number and types of activities psychologists judged to be necessary and they intend to actually perform. The lack of attention for causal analysis is remarkable as causal explanations are crucial to effective treatment planning.