In Chapter 2, we define mental illness and introduce readers to the DSM-5 as a diagnostic system. We outline diagnostic criteria for schizophrenia, bipolar disorder, major depressive disorder, and borderline personality disorder to present readers with the diagnoses of the 118 participants in our life story analyses. We discuss limitations of viewing mental illnesses as delimited categories, including studies showing high comorbidity, and review evidence that transdiagnostic approaches hold advantages. We conclude the chapter by providing rationales for approaching the two questions in our life story analyses from a transdiagnostic perspective.