Recent research conducted by independent investigators concerning the relationship between crime and narcotic (primarily heroin) addiction has revealed a remarkable degree of consistency of findings across studies. The major conclusion supported by the majority of these studies is that narcotic addicts commit a vast amount of crime and that much of this is directly related to the need to purchase drugs. A large proportion of the crimes committed does not consist merely of drug sales or possession, but involves other criminal behaviors including serious crimes. The strongest evidence of a causal relationship between narcotic drug use and crime is derived from longitudinal studies in which the amount of crime committed during periods of active addiction far exceeds that committed during periods of nonaddiction. Much of this crime goes unreported, although addicts, under conditions of strict confidentiality, have provided information that permits realistic estimates of criminal activity. Use of this methodology has permitted the identification of different types of addicts, especially with respect to the amounts and types of crimes in which they are engaged. The implication of these findings is that although addicts as a group commit a great amount of crime, they cannot be regarded as a homogeneous class. Some addicts commit many crimes, regardless of current addiction status, whereas others commit relatively few, and these are obviously related to their need to purchase drugs. There is a discernible impact of treatment on narcotic drug use and criminality. Although the relationships between addict characteristics and treatment response have yet to be fully determined, extensive prior criminal involvement is associated with a negative outcome.