OBJECTIVE To characterize the clients served by and the cats and dogs admitted to nonprofit spay-neuter clinics. DESIGN Cross-sectional survey. SAMPLE 2,154 dogs and 1,902 cats that were owned by 3,768 survey respondents and admitted to 22 nonprofit spay-neuter clinics across the United States between April 29, 2013, and January 24, 2014. PROCEDURES Participating clinics distributed surveys to clients during each of 4 quarterly study weeks. The survey collected descriptive information about clients' pets and households as well as their decision-making regarding sterilization of their pets. For each of the study weeks, clinics reported the total number of surgeries, including those involving shelter animals, feral cats, and other owned animals. RESULTS Respondents indicated that 49% of dogs and 77% of cats had not been examined previously by a veterinarian, except during vaccine clinics. Among animals ≥ 4 months of age, 1,144 of 1,416 (81%) cats and 572 of 1,794 (32%) dogs had not received a rabies vaccination. Previous litters were reported for 204 of 716 (28%) queens and 153 of 904 (17%) bitches. Most clients' (53%) household income was < $30,000 annually. Common reasons for clinic choice included cost; friend, neighbor, or family recommendation; and good reputation. CONCLUSIONS AND CLINICAL RELEVANCE Nonprofit spay-neuter clinics predominantly served low-income clients and animals lacking regular veterinary care, in addition to animals from shelters and community cats. These clinics increase access to services needed for animal population control and public health.