Crime, an act that breaks a law, and for which a punishment is provided by law. In a broader sense, the term is used to describe illegal activity in general. Juvenile delinquency refers to crime committed by persons under 18 years of age. Although any person who commits a crime is technically a criminal, this term is generally used to refer to a person whose entire life is patterned around crime. The scientific study of the causes and prevention of crime is called criminology. The branch of criminology dealing with the treatment of criminals is called penology.

Crime is a major social problem, especially in heavily populated areas. Police and other law-enforcement agencies are needed to detect and apprehend criminals and to protect citizens from robbery, violence, and other criminal acts. A system of penal or correctional institutions—jails, prisons, and reformatories—is maintained at public expense to punish or reform criminals or to withhold them from society.

Organized crime, in which gangs of criminals work together, reached its peak in the United States during the Prohibition era (1921-33) and has remained a problem ever since.