Forensic psychiatry is a subspecialty of psychiatry and is related to criminology. It encompasses the interface between law and psychiatry. According to the American Academy of Psychiatry and the Law, it is defined as "a subspecialty of psychiatry in which scientific and clinical expertise is applied in legal contexts involving civil, criminal, correctional, regulatory, or legislative matters, and in specialized clinical consultations in areas such as risk assessment or employment." A forensic psychiatrist provides services – such as determination of competency to stand trial – to a court of law to facilitate the adjudicative process and provide treatment, such as medications and psychotherapy, to criminals.

Forensic psychiatrists work with courts in evaluating an individual's competency to stand trial, defenses based on mental disorders (e.g., the insanity defense), and sentencing recommendations. The two major areas of criminal evaluations in forensic psychiatry are competency to stand trial (CST) and mental state at the time of the offense (MSO).

Competency to stand trial (CST) is the competency evaluation to determine that defendants have the mental capacity to understand the charges and assist their attorneys. In the United States, this is seated in the Fifth Amendment to the United States Constitution, which ensures the right to be present at one's trial, to face one's accusers, and to have help from an attorney. CST, sometimes referred to as adjudicative competency, serves three purposes: "preserving the dignity of the criminal process, reducing the risk of erroneous convictions, and protecting defendants' decision-making autonomy".

In 1960, the Supreme Court of the United States in Dusky v. United States established the standard for federal courts, ruling that "the test must be whether the defendant has sufficient present ability to consult with his attorney with a reasonable degree of rational understanding and a rational as well as factual understanding of proceedings against him." The evaluations must assess a defendant's ability to assist their legal counsel, meaning that they understand the legal charges against them, the implications of being a defendant, and the adversarial nature of the proceedings, including the roles played by defense counsel, prosecutors, judges, and the jury. They must be able to communicate relevant information to their attorney, and understand information provided by their attorney. Finally, they must be competent to make important decisions, such as whether or not to accept a plea agreement.

In England, Wales, Scotland, and Ireland, a similar legal concept is that of "fitness to plead".

Forensic psychiatrists are often called to be expert witnesses in both criminal and civil proceedings. Expert witnesses give their opinions about a specific issue. Often, the psychiatrist will have prepared a detailed report before testifying. The primary duty of the expert witness is to provide an independent opinion to the court. An expert is allowed to testify in court with respect to matters of opinion only when the matters in question are not ordinarily understandable to the finders of fact, be they judge or jury. As such, prominent leaders in the field of forensic psychiatry, from Thomas Gutheil to Robert Simon and Liza Gold and others have identified teaching as a critical dimension in the role of expert witness. The expert will be asked to form an opinion and to testify about that opinion, but in so doing will explain the basis for that opinion, which will include important concepts, approaches, and methods used in psychiatry.

Mental state opinion (MSO) gives the court an opinion, and only an opinion, as to whether a defendant was able to understand what he/she was doing at the time of the crime. This is worded differently in many states, and has been rejected altogether in some, but in every setting, the intent to do a criminal act and the understanding of the criminal nature of the act bear on the final disposition of the case. Much of forensic psychiatry is guided by significant court rulings or laws that bear on this area which include these three standards:

"Not guilty by reason of insanity" (NGRI) is one potential outcome in this type of trial. Importantly, insanity is a legal and not a medical term. Often, psychiatrists may be testifying for both the defense and the prosecution.