Existing jail studies rely on cross-sectional data drawn from samples in which shorter term offenders are underrepresented and longer term offenders are overrepresented. Studies need more complex approaches. They also need to include both those booked into jail and those present on any given day, because jails hold both sentenced offenders and inmates who are not convicted. The Illinois data were converted to rates per 10,000 population to permit comparisons among counties of varying sizes. Almost 20 percent of the counties have fewer than 6 cells per 10,000 population; about the same percentage have more than 15 cells. Ten percent of the jails turn over their populations 20 or fewer times per year, while 10 percent book 100 times their average daily populations. Patterns of jail use have little relationship to rates or patterns of crime. County population also has only a small association with jail use. Illinois jails are usually high or low in both bookings and holding periods. Crowding is most common in high-use jails. Some jails with low capacity rates are also crowded. 5 data tables and 16 references.