The analysis of the capacity of each Illinois facility was based on a systematic assessment of the types of offenders and the physical aspects of the facilities, as well as a scientific projection of growth patterns. The Illinois prison population experienced its most rapid growth between 1974 and 1985. Twenty-nine percent of the system's increased capacity is the result of double celling, especially at the four maximum-security prisons for adult males. The populations of these facilities are also the most violent. The age, size, noise levels, odors, heat, and bleak physical nature of these facilities contrast sharply with the modern prison designs of the last decade. Planned expansions of prisons will not keep up with the expected increase in the number of inmates. In addition, the current capacity ratings assume double celling and overstate the ideal capacity by nearly 4,000 beds. The State needs a realistic capacity plan to reduce the rated capacity at maximum-security and some medium-security facilities. Appendix explains population projections and gives data on each facility. Photographs, tables, and figures.