The Illinois adult prison population was as high in 1941 as it was in 1981, although there were only 7 adult institutions in 1941 versus 13 in 1981. Male felons comprise 95 percent of the prison population. The next largest group is the total female population, and the smallest is male misdemeanants. Although crime in Illinois generally increased in the early 1970's, the inmate population declined through 1974 when it reached its lowest levels of the entire period examined. Between 1975 and 1983, however, the adult prison population more than doubled to reach over 13,200. This increase is attributable both to new admissions from court and readmissions of parole and supervision violators. This is true for both males and females. One reason for the increased number of defaults was the enactment of statutory parole in 1973, which broadened the pool of prisoners released to supervision rather than simply discharged without supervision. Technical violations appear to account for a high number of readmissions, particularly at the end of the time series. The use of early or forced release as a means of reducing the prison population has kept the adult inmate population at a fairly stable level of 13,200 since 1982. The report discusses an additional data series: end-of-the-month supervision population, those people who have been released from prison but are still under Department of Corrections supervision. Graphs and five references are supplied.