This document provides an overview of abnormal psychology. It discusses how abnormality and mental disorders are defined and classified, which aims to allow researchers and clinicians to communicate about patients. Some of the key challenges in defining abnormality are discussed, including that there is no single behavior that defines it, but indicators include suffering, maladaptiveness, and statistical infrequency. The document also notes that mental disorders are common, with anxiety and mood disorders being the most prevalent. It emphasizes the importance of research in the field and describes different research designs used.