Clinical psychology is an integration of human science, behavioral science, theory, and clinical knowledge for the purpose of understanding, preventing, and relieving psychologically-based distress or dysfunction and to promote subjective well-being and personal development. Central to its practice are psychological assessment, clinical formulation, and psychotherapy, although clinical psychologists also engage in research, teaching, consultation, forensic testimony, and program development and administration. In many countries, clinical psychology is a regulated mental health profession.

The field is generally considered to have begun in 1896 with the opening of the first psychological clinic at the University of Pennsylvania by Lightner Witmer. In the first half of the 20th century, clinical psychology was focused on psychological assessment, with little attention given to treatment. This changed after the 1940s when World War II resulted in the need for a large increase in the number of trained clinicians. Since that time, three main educational models have developed in the US—the PhD Clinical Science model (heavily focused on research), the PhD science-practitioner model (integrating scientific research and practice), and the PsyD practitioner-scholar model (focusing on clinical theory and practice). In the UK and the Republic of Ireland, the Clinical Psychology Doctorate falls between the latter two of these models, whilst in much of mainland Europe, the training is at the masters level and predominantly psychotherapeutic. Clinical psychologists are expert in providing psychotherapy, and generally train within four primary theoretical orientations—psychodynamic, humanistic, cognitive behavioral therapy (CBT), and systems or family therapy.

Clinical psychology is different from psychiatry. Although practitioners in both fields are experts in mental health, clinical psychologists treat mental disorders primarily through psychotherapy. Currently, only six US states, Louisiana, New Mexico, Illinois, Iowa, Idaho, and Colorado allow clinical psychologists with advanced specialty training to prescribe psychotropic medications. Psychiatrists are medical doctors who specialize in the treatment of mental disorders via a variety of methods, e.g., diagnostic assessment, brief psychotherapy, psychoactive medications, and medical procedures such as electroconvulsive therapy (ECT) or transcranial magnetic stimulation (TMS). Additionally, psychiatrists are legally authorized to prescribe psychotropic medications in all states of the U.S. and in all provinces of Canada. In education, clinical psychologists attend a graduate institution and have a Doctor of Philosophy (Ph.D.) or a Doctor of Psychology (Psy.D.) degree, while psychiatrists completed their studies at a medical school and hold a medical degree (M.D.) or an osteopathic degree (D.O.), with the latter only available in the United States.