The U.S. Department of Justice defines crime as all behaviors and acts for which society provides formally approved punishments. Written law, both federal and state, defines which behavior is criminal and which is not. Some behaviors—murder, robbery, and burglary—have always been considered criminal. Other actions, such as domestic violence or driving under the influence of drugs or alcohol, were only recently added to the list of criminal offenses. Other changes in our society have also influenced crime. For example, the widespread use of computers provides new opportunities for white-collar crime, as well as adding a new word—"cybercrime"—to our vocabulary.

Two main government sources collect crime statistics. The Federal Bureau of Investigation (FBI) annually compiles the Uniform Crime Reports (UCR). The UCR collects data from about 17,000 city, county, and state law enforcement agencies, whose jurisdictions contain approximately 95 percent of the total U.S. population. The National Crime Victimization Survey (NCVS), prepared by the Bureau of Justice Statistics (BJS), bases its findings on an annual survey of 100,000 people.

Criminal behavior can range from actions as simple as taking chewing gum from a store without paying to those as tragic and violent as murder. Most people have broken the law, wittingly or unwittingly, at some time in their lives. Therefore, the true extent of criminality is impossible to measure. Researchers can keep records only of what is reported by victims or known to the police.