Case formulation links the client and his or her problems with the treatment.It captures both the strengths and the weaknesses in a complete summary of the client. The chapter introduces case formulation by first describing a behavioural case formulation and how a functional analysis can be conducted. The illustration of a behavioural formulation is then developed into a model that goes beyond identifying the antecedents and consequences of behaviours, and includes reference to the potential of mediation by thoughts and beliefs. Thus, a broader cognitive-behavioural model of case formulation distinguishes steps: (i) presenting problems, (ii) predisposing factors, (iii) precipitating variables, (iv) perpetuating cognitions and consequences, (v) provisional conceptualization, (vi) prescribed interventions and (vii) potential problems and client strengths. The chapter then provides two clinical examples of case formulation, beginning with a cognitive-behavioural case formulation then an example from the perspective of interpersonal psychotherapy. The chapter concludes with a discussion of transdiagnostic approaches to assessment and case formulation.