This review deals with studies dealing with the "quantification" of prostitution and reporting estimates of the number of persons who occasionally or regularly work on the streets of European countries. The estimates also concern the number of foreign prostitutes. These estimates are drawn from accounts of privileged witnesses: social workers, non-governmental organisation volunteers engaged in informing or helping prostitutes, police reports. Indoor prostitution is much more difficult to quantify, although there are some studies which examine the number of advertisements published in the press. In general, research has shown that in Italy the preceding decline in prostitution has been reversed from the Eighties to the Nineties due to mass immigration. This new trend has not involved other European Union countries, which have had very different experiences: at one extreme, the Netherlands has had a constant growth in all types of prostitution and in its foreign component; at the other, Sweden has experienced a decline in prostitution, in which foreigners play a very small role.