We know far too little about what really happens inside prisons. In this issue of eco, we look beyond the walls of our penitentiary system and let the data speak. The overwhelming majority of inmates serve their sentences in severely overcrowded facilities. This is a chronic problem that calls for structural interventions, including the construction of new prisons. It is not only a matter of upholding constitutional principles in the treatment of those who have committed crimes, but also of ensuring greater safety for society as a whole. Overcrowding and recidivism are in fact two sides of the same coin: a purely punitive prison risks becoming a training ground for crime. By contrast, where detention conditions are better, inmates can take part in work and rehabilitation programs that support reintegration and significantly reduce the likelihood of reoffending.