The common marmoset (Callithrix jacchus) is a small New World primate that is native to eastern Brazil and has been used in biomedical research since the early 1960s. Use of this species for research purposes continues to grow at a rapid pace as they are a viable alternative to other nonhuman primate species. We discuss clinical care, such as preventative medicine, anesthesia, and routine clinical procedures. Important viral, bacterial, parasitic, and gastrointestinal tract diseases are discussed, with relevant details on cause, transmission, pathologic changes, diagnosis, and treatment. Current problems affecting marmoset health and research in the modern vivarium are emphasized.