Between January 1975 and May 1995 the number of French prison inmates rose from 26,032 to 55,479. The last time there were so many prisoners was in 1948, when one-third of all inmates were in prison for having collaborated with Nazi Germany. The factors contributing to this growth in the prison population were analyzed in two different but complementary ways. The first involved examining the socio-demographic and criminal characteristics of the inmates, which requires a differential analysis of the growth in the number of inmates. The data were examined in terms of penal category (which involves studying remand prisoners), the structure of convicted prisoners according to sentence length and type of principal offense, and the structure in terms of nationality (French and foreign). The second way examined prison entry flows and detention length to determine whether the increase in prison population resulted from an increase in the flow of entries (the detention length being stable or falling), sentence length (the flow of entries being stable or falling), or both. Both analyses used the SEPT (SEries Penitentiaires Temporelles) database. Notes,