Homelessness is a significant social problem in the United States, with an estimated 2.5 million homeless people in this country today. While criminal activity may become a means for the homeless to obtain resources needed for basic survival, little is known about the level of criminal activity among the homeless or about the types of crimnal activity in which the homeless engage. Although there are many ways in which the criminal behavior of the homeless may be assessed, each has its own methodological problems. Existing research indicates that substantial numbers of the homeless have a history of involvement in the criminal justice system and that the homeless may be overrepresented among certain identified criminal groups. A study was conducted to gather information through self-report and from archival data on a sample of 125 homeless shelter users concerning both their criminal history and their current illegal activity. Interview data were gathered on prior arrests, incarceration history, illegal drug use, and current illegal sources of support. The results revealed a wide range of past and current criminal behavior: as many as 62.4% of the subjects had been arrested for illegal behavior, or admitted to earning current illegal income, and 44.3% of male respondents had a history of incarceration in jail or prison. Criminal behavior appeared to serve various functions among the homeless, and the homeless who engaged in illegal behavior can be classified as chronic criminals, supplemental criminals, criminals out of necessity, substance abusers, or the mentally ill. While the homeless as a whole engage in relatively high levels of illegal activity, for many this is an adaptive response to dealing with severely limited resources. (NB)