Legal psychology involves the application of empirical psychological research to legal institutions and people who come into contact with the law. Legal psychology is a field which takes basic social and cognitive theories and principles and applies them to issues in the legal system such as eyewitness memory, jury decision-making, investigations and interviewing.

Legal psychology and forensic psychology together form the field more generally recognized as "psychology and law". Following up on earlier efforts by psychologists to address legal issues, psychology and law became a field of study in the 1960s as part of an effort to enhance justice. The American Psychological Association's Division 41, the American Psychology-Law Society, is active, and there are similar societies in Britain and Europe.

There are several legal psychology journals, including Law and Human Behavior, Psychology, Public Policy and Law, Psychology, Crime, and Law, and Journal of Psychiatry, Psychology and Law. In addition, research by legal psychologists is regularly published in more general journals that cover both basic and applied research areas.
Legal psychologists typically hold a Ph.D. in some area of psychology (e.g., clinical psychology, social psychology, cognitive psychology, etc.), and apply their knowledge of that field to the law. Although formal legal training (such as a J.D. or Master of Legal Studies degree) can be beneficial, most legal psychologists hold only the Ph.D. In fact, some argue that specialized legal training dilutes the psychological empiricism of the researcher. For instance, to understand how eyewitness memory "works," a psychologist should be concerned with memory processes as a whole, instead of only the aspects relevant to the law (e.g., lineups, accuracy of testimony).

A growing number of universities offer specialized training in legal psychology as either a standalone Ph.D. program or a joint J.D. / Ph.D. program. A list of American universities that offer graduate training in legal psychology can be found here on the website of the American Psychology-Law Society
Many legal psychologists work as professors in university psychology departments, criminal justice departments or law schools. Like other professors, legal psychologists generally conduct and publish empirical research, teach various classes, and mentor graduate and undergraduate students. Many legal psychologists also conduct research in a more general area of psychology (e.g., social, clinical, cognitive) with only a tangental legal focus. Those legal psychologists who work in law schools almost always hold a J.D. in addition to a Ph.D.

Psychologists specifically trained in legal issues, as well as those with no formal training, are often called by legal parties to testify as expert witnesses. In criminal trials, an expert witness may be called to testify about eyewitness memory, mistaken identity, competence to stand trial, the propensity of a death-qualified jury to also be "pro-guilt," etc. Psychologists who focus on clinical issues often testify specifically about a defendant's competence, intelligence, etc. More general testimony about perceptual issues (e.g., adequacy of police sirens) may also come up in trial.

Experts, particularly psychology experts, are often accused of being "hired guns" or "stating the obvious." Eyewitness memory experts, such as Elizabeth Loftus, are often discounted by judges and lawyers with no empirical training because their research utilizes undergraduate students and "unrealistic" scenarios. If both sides have psychological witnesses, jurors may have the daunting task of assessing difficult scientific information.

Psychologists employed at public policy centers may attempt to influence legislative policy or may be called upon by state (or national) lawmakers to address some policy issue through empirical research. A psychologist working in public policy might suggest laws or help to evaluate a new legal practice (e.g., the use of sequential lineups v.s. simultaneous lineups).
Trial consulting
Some legal psychologists act as trial consultants. No special training nor certification is needed to be a trial consultant, though an advanced degree is generally welcomed by those who would hire the trial consultant. The American Society of Trial Consultants does have a code of ethics for members, but there are no legally binding ethical rules for consultants.
Some psychologists who work in academics are hired as trial consultants when their expertise can be useful to a particular case. Other psychologist/consultants work for or with established trial consultant firms. The practice of law firms hiring "in-house" trial consultants is becoming more popular, but these consultants usually can also be used by the firms as practicing attorneys.
Trial consultants perform a variety of services for lawyers, such as picking jurors (usually relying on in-house or published statistical studies) or performing "mock trials" with focus groups. Trial consultants work on all stages of a case from helping to organize testimony, preparing witnesses to testify, picking juries, and even arranging "shadow jurors" to watch the trial unfold and provide input on the trial. There is some debate on whether the work of a trial consultant is protected under attorney-client privilege, especially when the consultant is hired by a party in the case and not by an attorney.
Advisory roles
Legal psychologists may hold advisory roles in court systems. They may advise legal decision makers, particularly judges, on psychological findings pertaining to issues in a case. The psychologist who acts as a court advisor provides similar input to one acting as an expert witness, but acts out of the domain of an adversarial system.

Forensic psychology is the application of psychological principles and knowledge to various legal activities. Typical issues include child custody disputes, child abuse or neglect, assessing personal capacity to manage one's affairs, matters of competency to stand trial, criminal responsibility, personal injury, and advising judges in matters relating to sentencing regarding various mitigants and the actuarial assessment of future risk.

Forensic psychologists typically hold a Ph.D. or Psy.D. in clinical psychology. In the United States, in order for a psychologist to practice as a forensic psychologist, the individual must be a state licensed psychologist and receive certification as a Diplomate in Forensic Psychology by the American Board of Forensic Psychology. To qualify, the psychologist must have at least 4 years of postdoctoral experience in forensic psychology, 100 hours of specialized training in forensic psychology, and pass an oral and written examination.

While some Masters level programs in forensic psychology do exist, it is not possible for those individuals to practice legally as forensic psychologists in the United States or Canada as a doctoral degree is required for licensure as a psychologist.

In other countries, training and practitioner requirements vary. For example, in the United Kingdom, a person must obtain the Graduate Basis for Registration with the British Psychological Society--normally through an undergraduate degree. This would be followed by Stages 1 (academic) and 2 (supervised practice) of the Diploma in Forensic Psychology (which would normally take 3 years full time and 4 years part time). Assessment is via examination, research, supervised practice and the submisssion of a portfolio showing expertise across a range of criminological and legal applications of psychology. Once qualified as a "Chartered" psychologist (with a specialism in forensic psychology), a practitioner must engage in Continued Professional Development and demonstrate how much, of what kind, each year, in order to renew his/her practising certificate.

Forensic psychologists serve a number of roles in support of and within the criminal justice system. When an individual is believe to be incompetent to proceed to trial, a forensic psychologist or forensic psychiatrist is brought in to examine and assess the individual in custody. Based on the assessment, a recommendation is made to the court as to the individual's competence to proceed to trial. Forensic psychologists are also called upon to examine individuals who claim an insanity defense, referred to in various states as Not Guilty by Reason of Insanity (NGRI or NGI) or Not Guilty due to Mental Disease or Defect. Additionally, forensic psychologists are frequently asked to make an assessment of an individual's risk of re-offending or dangerousness. They also provide information and recommendations necessary for sentencing purposes, grants of probation, and the formulation of conditions of parole. Forensic psychologists are routinely called upon in death penalty cases to provide analysis of the intentions, motivations and personality characteristics of the accused. In juvenile courts, they often are asked to help determine whether a youthful offender can be rehabilitated. They also occasionally assist prosecutors, defenders, and law enforcement investigators in understanding a range of normal and criminal behaviors, sometimes serving as "criminal profilers".

Once an individual has been found not competent to proceed to trial or Not Guilty by Reason of Insanity, forensic psychologists will work within a facility setting to help the individual to either regain competence so the person is able to stand trial or will provide treatment until such time as the individual is deemed no longer a risk to society or themselves.

In the civil law arena, forensic psychologists often provide assessments of whether someone has been harmed by some event. For example, in a wrongful death suit, a psychologist might offer opinions as to whether a plaintiff suffered emotional trauma in response to the death of a loved one. They might also assess the emotional injuries suffered by someone who has been injured in an accident or who witnessed a traumatic event. Psychologists are often called upon in sexual harassment suits to describe the impact of the harassment on the purported victim. In this arena, the forensic psychologist might be required to provide treatment recommendations or to analyze the specific treatment needs of an individual, and might be asked to determine the potential cost of such treatment.

In the arena of workers' compensation law, a forensic psychologist might be called upon to describe how workplace stress factors impacted the psychological functioning of a claimant, or to determine whether the purported work place stress had any effect on the worker at all. As in the more general civil law context, the forensic psychologist might be asked to determine treatment needs and treatment plans.

In the family law arena, forensic psychologists are often called upon to assess the "best interests" of children whose parents are divorcing. Commonly, this involves making recommendations to a Court with respect to child custody arrangements. Child custody mediation is another role that forensic psychologists undertake in the family law arena—serving as a mediator between divorced parents who remain in dispute about the needs and interests of their children. In some jurisdictions, forensic psychologists are appointed as "special masters" by the court, and are charged with making both recommendations and orders for the care of children in disputed custody situations.