This article outlines a systematic approach to the formulation of clinical problems for practitioners working with children and their families. Whilst assessments in child and adolescent mental health often use a range of theoretical and practical approaches for data collection, there are relatively few resources to assist the clinician in integrating this information to develop a formulation that leads to a well considered intervention plan. The formulation approach presented here was designed to assist in training child and adolescent clinicians in a method that facilitates the process of understanding complex cases. This is done by examining the patterns of strength and difficulty identified during an assessment and systematically providing an explanation for these using ;the Four Ps' - predisposing, precipitating, perpetuating, and protective factors - to structure the clinician's thinking. Interventions that address patterns of strength and difficulty for each of the ;Four Ps' are recommended when working with complex clients. This formulation approach may take more time than merely summarizing the case, but the benefits are a more comprehensive understanding of the client. This means that problems often associated with working with complex cases can be identified and addressed, reducing the risk of drop out, poor engagement or treatment failure.