This Element reviews and augments research on changes over time in U.S. crime rates during the past several decades. Major topics include the data sources for studying crime trends; the relationship between homicide rates and rates of property crime, imprisonment, and firearm availability; trends in crime by sex, race, and age; the relationship between crime trends and economic conditions; crime trends and social institutions; abrupt changes in crime rates and exogenous shocks; forecasting crime rates; and the future of crime trends theory and research. The study of crime trends is as intellectually rewarding and practically important as any topic in criminology. But attracting scholars to this field of study of crime trends will require significant advancements in theory, methods, and policy application.