This chapter focuses on mice and rats, and provides detailed information that may be useful for veterinarians treating these animals. Mice are continuous, polyestrous rodents that should be bred in polygamous or monogamous setups because of the males' aggressive territoriality behavior. When breeding mice that have been housed in a polygamous ratio, there may be one male with two to six females. Females are removed from a polygamous cage before parturition, whereas the monogamous pair is maintained together with the young until weaning. Mice are maintained in environments that are similar to other small rodents but require a thorough cleaning of their cage more often because of their malodorous urine. Ventilation is essential for small rodent housing to prevent irritation of the respiratory tract from ammonia vapors generated by urine. Quarantining is important when a new animal is being introduced into a setting in which there is an established group. As with other animals, a 30-day quarantine period is recommended, along with a physical examination and fecal parasite check. To maintain oversight of breeding animals' health and reduce the exposure of young animals to infectious disease and parasites, routine screening of representative animals within the colony is recommended. In very large colonies, special caging, food, and water may be necessary to prevent exposure to disease organisms.