Forensic psychology is the result of the field psychology being applied to the field of Criminal Justice. In other words, it is the process of a forensic psychologist examining the psychological perspectives of human behavior and applying them to the legal system.

The origins of the field of forensic psychology can be traced back to the beginning of the 20th century. In 1901, William Stern studied memory by asking his students to view a picture for 45 seconds. He than asked them to describe what they had seen. He continued the experiment by varying the degree of time the picture could be viewed. This research questioned the reliability of eyewitness testimony in court. Tests such as this, and others that came after has led to the increasing uses of the application of psychology in law and law enforcement. Today it is a well-established field in the courtroom and in law enforcement. A forensic psychologist performs many different duties within the Criminal Justice field.

The most important duty of a forensic psychologist is to act as an expert witness. This involves testifying in court, the ability to reformulate psychological findings to fit within the language of the legal system, and the ability to provide this information to legal personnel in a manner they will comprehend.

Before the trial begins, they verify if a defendant is competent to stand trial. This involves an evaluation of the defendant's sanity or insanity at the time they committed the offense. They may evaluate an individual's risk of re-offending, or becoming a danger to others or themselves.

Forensic psychologists may work with the Public Defender, the States Attorney, or private attorneys. They provide sentencing recommendations and treatment recommendations for offenders as well as evaluating the credibility of any witnesses. They are sometimes involved in the selection of jury members.

A forensic psychologist may be asked to help a judge determine what parent should receive custody of a child, or to determine if an accident victim has sustained either psychological or neurological damage. They may be asked to counsel the victims of a crime, and help them prepare to testify along with learning to cope with emotional stress.

Forensic psychologists aid in the training and evaluation of police and other law enforcement personnel, and provide criminal profiles when needed.

Many forensic psychologists chose to focus their careers on research. This may range from the examination of an eyewitness testimony to developing and improving methods used during an interrogation.

Another area of Forensic Psychology is public policy. Researchers help with the designing of correctional facilities and prisons. They may also counsel inmates and probationers.