Prevailing classification systems backed by the American Psychiatric Association (DSM) and World Health Organization (ICD) generally assume that mental health conditions are best represented by discrete entities that are qualitatively distinct from one another and from mental health. These diagnostic categories, such as major depression, antisocial personality disorder, and obsessive-compulsive disorder, are ubiquitous in research, clinical, training, legal, and public health contexts. Yet they are out of sync with a vast quantity of scientific data on the presentation, causes, and treatments of psychopathology. Empirically speaking, categorical diagnoses tend to be unreliable (over time and across reporters), have fuzzy boundaries with one another and mental health, do not capture many clinical presentations encountered in routine practice, and lack distinctive causes and recommended treatments. Dimensional perspectives recognize the same signs and symptoms as categorical rubrics, but they do not shoehorn them into categories. Instead, mental health conditions are conceptualized as a profile of scores on psychopathological dimensions, which express individual differences as a matter of degree, not kind. Such dimensions might refer to fine-grain psychopathology symptoms, broad-bandwidth personality features, neurobiological systems that confer vulnerability to mental illness, and other factors. Quantitative differences on these dimensions, relative to one's peers or to established benchmarks, can be used to characterize patients' presenting problems, make clinical decisions, and investigate the causes and consequences of psychopathology. This book maps the landscape of dimensional approaches to psychopathology. It contrasts dimensional views, which vary significantly in scope and structure, to each other and to categorical frameworks, describing a range of potential research and clinical advances, highlighting recent developments in basic research across diverse biological and social approaches to mental health. It ends with prominent questions and challenges facing dimensional conceptualizations, and the scientific and political achievements that are needed for them to compete with, and possibly replace, categorical models.