Including persons booked prior to trial and jailed after conviction, there were approximately 1.1 million California jail admissions in 1983, and the average length of stay was 14.2 days. The jail population in California has nearly doubled in the last decade, with particularly dramatic increases in the last 4 years. Jail populations will continue to rise for the rest of 1980, but the increase will probably begin to slow, producing a 20-percent increase by 1990. An increased number of felony arrests in concert with the more stringent processing of such arrest are major factors in jail population increases. The majority of pretrial jail admissions (70-85 percent) are for misdemeanors. Since about 80 percent of these persons are released within hours, at any given time, approximately 76 percent of jail inmates are charged with felonies. Slightly more than half of the jail population consists of sentenced offenders; the remainder are pretrial detainees. Jail inmates are typically 18-30 years old, single, and unemployed. 18 tables and appended supplementary data.