This presentation will trace the evolution of diagnostic reasoning in psychiatry from the pre DSM III era to the present and examine the implications of current efforts to shift from categorical diagnoses toward more dimensional and transdiagnostic approaches. It will review contemporary conceptual models for assessing anxiety, depression, and trauma to illustrate how different diagnostic frameworks influence prevalence estimates, genetic findings, clinical trial outcomes, and, importantly, the ways we communicate with the public about mental health. Finally, the presentation will introduce a method of case formulation designed to address challenges in current assessment and treatment practices, including the all too common practice of accumulating comorbid diagnoses leading to ‘polytreatment’ approaches.