Desistance should be the main ground for reentry policies for imprisoned offenders. However, theories on desistance are diverse, and they disagree about the key factors related to the origin, maintenance, and failures of the desistance process. This research considers three main theories of desistance-control, cognitive transformation, and strain-social support-to explain desistance in a qualitative sample of imprisoned men in Spain. The main finding of the research is that strain-social support theory may be of primary importance for understanding desistance because of its capacity to explain processes of change that begin during imprisonment and that continue upon release.