Despite considerable evidence that certain life-course transitions can play a significant role in helping some offenders abandon crime, several fundamental issues remain unresolved. In this dissertation, I examine the links between crime and two lifecourse transitions related to the development of families in adulthood: cohabitation and marriage. Using data from the National Youth Survey (NYS), I investigate the extent to which both types of relationships can contribute to desistance. I then evaluate the major theoretical mechanisms through which marriage is most likely to promote behavioral change. Finally, I examine the degree to which these relationships foster desistance for both men and women. Results indicate that marriage has the capacity to promote desistance, whereas cohabitation does not, and that the effects of marriage on crime are conditional on both the social orientation of the spouse and the quality of the marital relationship. These and other results are mostly consistent with social control and social learning theories of crime and desistance. In addition, the results of the analyses indicate that the effects of marriage on crime are similar among men and women.