Criminal investigation is an applied science that involves the study of facts that are then used to inform criminal trials. It is an ensemble of methods by which crimes are studied and criminals apprehended. A complete criminal investigation can include searching, interviews, interrogations, evidence collection and preservation, and various methods of investigation. Modern-day criminal investigations commonly employ many modern scientific techniques known collectively as forensic science.

Criminal investigators seek to ascertain the methods, motives, and identities of criminals and the identity of victims and may also search for and interrogate witnesses. Criminal-investigation departments compile data, as well as lists of stolen and lost property, and have ready access to such public records as automobile and firearms registrations and such private records as laundry and dry-cleaners’ marks, pawnshop and secondhand-dealers’ transactions, and many more. Interrogation of suspects is one of the most important functions of criminal investigation.

Guidelines for investigating specific crimes show investigators how to classify and define these crimes, deal with the special problems each crime presents, prove the elements of the offense, and collect evidence, especially from victims, witnesses, and suspects. Investigations of robbery, assault, sex offenses, homicide, burglary, larceny/theft, fraud, white-collar crimes, motor vehicle theft, arson, organized crime, and drug-related crime are all treated individually.

Criminal investigators are highly trained and have access to a wide variety of tools including networks of investigators, surveillance equipment, and records checks. Criminal investigators also adhere to laws and regulations surrounding investigations to ensure that information is obtained in a legal manner which aids in court.

Criminal investigation is a multi-faceted, problem-solving challenge that involves the collection and analysis of evidence. It is a broad term encompassing a wide range of specialties that aim to determine how events occurred and to establish an evidence-based fact pattern to prove the guilt or innocence of an accused person in a criminal event.