This paper studies the use of mobile crisis teams—a non- uniformed pair consisting of a mental health worker and a medic—as either a substitute or complement to traditional police units responding to 911 calls involving mental illness, homelessness, addiction, and other non-dangerous situations. We evaluate the longest-running mobile crisis response program in the U.S., Crisis Assistance Helping Out on the Streets (CAHOOTS) in Eugene, Oregon. We find that a series of expansions of CAHOOTS into new areas and times reduced the likelihood that a 911 call resulted in an arrest, and had the long-term benefit of reducing the number of follow-up 911 calls from the same address. However, CAHOOTS often responds to the same calls as the police, acting as complement rather than a substitute. The reduction in arrests likely reflects CAHOOTS’ role in de-escalating tense situations and resolving incidents without coercive measures. After the initial expansions, additional CAHOOTS capacity is used mostly for calls that would otherwise go unanswered. We conclude that crisis response teams play an important role as a complement to the police, rather than only acting as substitutes. Further expansions are likely to have only limited effects on reducing police responses