This article examines the evolution of prison populations in Western Europe from 1982 to 2011 and its relation with recorded crime trends in the region. Data are taken mainly from the Council of Europe Annual Penal Statistics in the case of prison statistics and the European Sourcebook of Crime and Criminal Justice Statistics in the case of police and conviction statistics, both complemented with the Nordic Criminal Statistics and Eurostat Crime Statistics. The results show that prison populations rates (stock) rose constantly until 2005 and seem relatively stable since then. On the contrary, the annual flow of entries into penal institutions has decreased almost continuously since 1987. This apparent paradox is explained by the fact that the average length of detention has steadily increased during the whole period under study. In brief, less people are sent to prison each year, but they remain in prison for longer periods of time. The upward trend in the average length of detention is related to the development of tough on crime policies across Western Europe and to the increase of drug offences and non-lethal violent crime until the mid-2000s. In that context, an analysis by offence shows similar trends in police, conviction, and prison statistics. These results falsify the hypothesis of total independence between crime trends and imprisonment rates. They also suggest that the deterrent effect of imprisonment has often been overestimated, and they cast a shadow on the validity of criminological theories that place property as the main cause of crime.