California jail populations have nearly doubled in the last decade, with increases being particularly dramatic in the last 4 years. Currently there are approximately 9,000 more prisoners than jails are designed to hold. Recent jail population increases have resulted both from more bookings and longer stays. Causes of increased jail populations include more people in the State, criminal law and criminal justice policy changes, increasing arrest rates, and more stringent law enforcement processing of felony arrests. Jail populations will continue to rise for the rest of the 1980's, but at a slower pace, increasing current jail populations by about 20 percent by 1990. On an average day, just under half of the jail population is in pretrial custody. Inmates are typically 18-30 years old, single, and unemployed. Appended supplementary information, 18 tables, and 2 figures.