This paper analyses the situation of the foreign population held in European penal institutions and its evolution during the last 20 years. The analyses were based on data taken from the Council of Europe's Annual Penal Statistics (SPACE) that provides information on the characteristics of prison populations in nearly all European countries. The analyses are focused on European Union (EU) member states, taking into account the moment when they joined the EU, but they include also references to non-member states. After drawing comparisons between the numbers of foreign prisoners across Europe, the paper studies the relationship between penitentiary data (rates of foreign prisoners in the total prison population and per 100,000 inhabitants) and demographic indicators for each country (percentage of foreign people in prison in comparison with the percentage of the foreign people in the total population). Finally, it analyzes the link between indicators such as the economic level or the geopolitical situation of the country, and the number of foreign inmates. (Published Abstract) Figures and references