Prison populations in these countries have faced similar evolutions since the 1980's. The presence of such general tendencies show, however, that explanations for changing prison populations should be found in broad, structural changes that occurred in these western countries, but with attention paid to particular national differences. In the mechanisms that influence prison populations, the authors have identified factors external to the criminal justice system (demography and economy), internal factors (criminal justice policy), and "interfering" factors (public opinion and politics). The influence of each separate factor depends on its interaction with the other factors. The criminal justice system, however, appears to be a pivotal point in these interactions. Although the system cannot influence demographical and economical trends, prison populations are apparently dependent on how the criminal justice system reacts to these external factors. The importance of politics and of criminal policy are obvious in a comparison of countries with increasing prison population with those having a decreasing prison population. Expansionist policies are characterized by confidence in the advantages of imprisonment, a constant increase of the prison population, prison overcrowding, and expansion of prison capacity and of prison staff. The discussion shows that even if the criminal justice system cannot influence all elements that determine its functioning, it has sufficient leeway to refute the argument that changing prison populations are a pure product of external factors out of its reach. 153 references