What's the difference between a psychologist and a psychiatrist?

First is training -- a psychologist has a Ph.D., Ed.D. or Psy.D., a psychiatrist has an MD. Psychologists are typically trained in research, assessment/testing, and therapy/treatment. Psychiatrists are trained in general medicine, psychopharmacology, psychodiagnostics and therapy/treatment. Both are qualified to do therapy. In most states psychologists have very limited prescription privileges if any, while an MD can prescribe the full slate of psychiatric medications. Psychologists are trained and able to do formalized assessment using standardized instruments of a wide variety of issues including psychopathology, neuropsychological issues, learning issues, vocational assessment etc. --  this is typically not the purview of a psychiatrist.
Roy Huggins
Put simply, a psychiatrist is a medical doctor (MD) with a specialization in mental health and a psychologist is a person with post-graduate training, usually at the doctoral level, in psychology.

Psychiatrists may perform psychotherapy (aka talk therapy) but modern psychiatrists often do not. Psychiatrists often provide medical treatment for mental illness; most notably they prescribe and manage medications. They may discuss other behavioral/medical treatments, as well.

Psychologists actually may work in a number of fields besides just the treatment of mental health. For example, psychologists can be found in human resources departments, safety planning teams, schools and various research contexts. A clinical psychologist or counseling psychologist works with people on mental health issues through psychotherapy of some kind. In some states, psychologists can prescribe medication but that is not a standard across the country.
A psychiatrist receives a doctorate in medicine, then spends an additional four years in psychiatric residency studying the diagnosis and treatment of mental illness. Trained in mostly a biomedical perspective, psychiatrists treat mental illness primarily with medication. Psychologists pursue a doctorate in psychology, either receiving a Ph.D., Psy. D. or Ed.D. in their specialty. Internship and supervision generally add another 3-4 years post doctorate for licensure. Trained in developmental theory, psychopathology and neurobiology, Psychologists generally work via a form of talk therapy for dealing with mental illness. In some states, psychologists can prescribe medication. A easy way to make this distinction is that psychiatrists generally treat symptoms and disorders with medication, while psychologists treat with talk therapy.
The difference between a psychologist and a psychiatrist is this. They can both provide counseling/therapy, they can both get licenses by the state, but a psychiatrist is actually an MD who has specialized in psychology. This means that a psychiatrist can prescribe medications for people needing them for psychological issues. 

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