Comorbidity refers to the situation in which an individual qualifies for the diagnosis of more than one disorder, either across their lifecourse (lifetime comorbidity) or at present (current comorbidity). In many services, comorbidity of psychological disorders is the norm rather than the exception. In this chapter, the common experience of comorbid problems is described and the literature on the treatment of comorbidity using CBT is briefly reviewed. A transdiagnostic model of psychological distress is introduced that unifies across different cognitive, behavioural and interpersonal processes – proposing that they each involve control as a process. A simple model for formulation is introduced that allows therapists to model the clients’ conflicting attempts to control their internal experiences, and the underlying personal goals that drive these processes. Several cases are introduced to illustrate how a CBT approach can be based around a theory of hierarchical control and conflict. The approach is designed to guide and sustain clients’ present moment awareness on the higher order goals and values that drive their experience and allow them to shift perspectives and reprioritise in a way that gains a greater overall level of control and purpose in their lives. A newly formed version of cognitive therapy called Method of Levels is proposed as the most efficient technique, yet its aims are consistent with many familiar methods in CBT. The chapter finishes with key points for discussion in relation to diagnosis, its utility, and the future of the transdiagnostic approach.