--- - |2 Although a number of manuals describe how to develop a cognitive‐behavioural case formulation (CBCF) to assist the clinician in developing and implementing tailored CB treatment for complex or comorbid cases, methods to evaluate and test functional hypotheses in the formulation for a particular case are not widely known or used. Thus, formulation‐based, tailored treatments for such cases are likely to be guided by case formulations of uncertain validity. The present article presents guidelines and choices available to the clinical trainee, researcher, or practitioner planning to use a person‐specific (PS) approach to testing CBCF hypotheses about functional relations between triggers, cognitions, and distress or dysfunction. This approach involves repeated assessments completed by the client using an individualized questionnaire that includes both nomothetic and idiographic items of greatest relevance to the client. Guidelines and options for developing this questionnaire and for data collection within the context of a collaborative therapist–client relationship are described. PS data analyses are then used to test case formulation hypotheses that vary in complexity from simple bivariate relations to complex dynamic mediation. A case example using this PS approach is described. Simpler options for the PS evaluation of a CBCF usable by practitioners and trainees as well as more complex options for clinical researchers are included. - Clinical Psychology & Psychotherapy, Volume 25, Issue 5, Page 672-691, September/October 2018.