California is one of the early adopters of extended foster care. The California Fostering Connections Act (AB12) was signed into law on September 30, 2010. The law ensured that, starting January 1, 2012, eligible foster youth had the right to stay in care until their 21st birthday. In addition to having the largest state foster care population in the US, California's county-administered foster care system draws the attention of many other states to California's implementation of extended care (Courtney, Charles, Okpych, Napolitano, & Halsted, 2014). Leveraging youth survey data and administrative data, this memo investigates youth- and system-level factors associated with the length of time that foster youth in California remain in out-of-home care after their 18th birthday. Understanding factors associated with youths' extended foster care (EFC) participation is important, not only for predicting future demands for the program, but also for identifying subgroups that may be excluded from extended care. The authors find that youth characteristics are associated with youths' stays in care past their 18th birthday, but the change in state policy that occurred in 2012 and between-county variation in participation in EFC play much larger roles.