Ambulatory surgery centers (ASCs) are rapidly growing as a setting for the delivery of health care services, both in California and in the United States. At present, little is known about either the measurement of quality in the ASC setting or about the regulation of ASCs by the various U.S. state governments. This report describes the California ASC landscape, compares California ASCs to hospital outpatient surgery departments, and compares California's regulation of Ambulatory Surgery Centers to the regulations of other states. The research revealed differences in patterns of volume and procedure specialization between ASCs and hospital outpatient surgery departments. These differences could have implications for quality if high volume is associated with quality or if more specialization is associated with higher quality. However, there is little research on these questions for outpatient surgery at present.