The Acjachemen are an Indigenous people of California. Published maps often identify their ancestral lands as extending from the beach to the mountains, south from what is now known as Aliso Creek in Orange County to the Las Pulgas Canyon in the northwestern part of San Diego County. However, sources also show that Acjachemen people shared sites with other Indigenous nations as far north as Puvunga in contemporary Long Beach.

Gerónimo Boscana was an early 19th-century Franciscan missionary in Spanish Las Californias and Mexican Alta California. He is noted for producing the most detailed ethnographic picture of a Native Californian culture to come out of the missionary period, an account that "for his time and profession, is liberal and enlightened".