For more than two decades, the validity and reliability of official statistics on crime have been treated as highly questionable. Recently a number of studies have investigated the construction of official statistics on crime and issues related to their reliability and validity. However, there has been no recent attempt to bring together and evaluate these studies; thus, we do not know what official statistics mean and how we should interpret them. In the present paper it is argued that it is now possible to develop an understanding of what official statistics measure. It appears that both citizens and the police are in general agreement about what a serious crime is: it involves bodily injury (or serious threat of bodily injury), the property stolen is of high value, the act is committed by a stranger, or it involves breaking and entering. The authors argue that the perceived seriousness of the crime, first and primarily as defined by the victim, second as determined by the police, apparently accounts for most of the variance in whether a crime is reported and officially recorded; personal characteristics of the offender and victim have only minor effects. It is concluded that the Uniform Crime Reports (UCR), as measured by the FBI Index of Crime, are valid indicators of serious crimes as defined by the citizenry. The evidence supporting this conclusion is very substantial with regard to motor vehicle thefi, robbery, burglary, and homicide, while with larceny, rape, and aggravated assault, the evidence supporting this conclusion is substantial but does require the interpretation of conflicting evidence.