The historical origins of the case formulation approach to assessment and treatment are described and its role in clinical practice and research discussed. It is argued that treatment based on individual case formulations should not be precluded from clinical trials. The empirical evidence for the reliability and efficacy of case formulation is reviewed. The evidence that an idiosyncratic case formulation approach to treatment has any advantage over a standard protocol is equivocal; however, the studies that have been carried out are under powered and potentially suffer from a Type II error. The standard procedure for case formulation is briefly described and the argument advanced that this method of clinical assessment should be soundly based upon empirical evidence and hypothesis testing and not on speculation. Three modifications are suggested to the current practice of case formulation. These are: 1) the conceptualization of dysfunctional systems in the maintenance of clinical problems; 2) the historical background of a clinical problem should be described in terms of vulnerabilities and epidemiological evidence-base; and 3) the pivotal role of social behaviour and context should be emphasized and accommodated in a formulation. The advantages of a case formulation, in providing an understanding of the maintenance of clinical problems, in providing an integrated approach to intervention are outlined.