This chapter provides an overview of the biology and medical care of hamsters and gerbils. The golden hamster is the largest of the common pet species, with an average body weight of 120 g. Other hamster species commonly found in the pet trade are the Chinese hamster and the Siberian or dwarf hamster. Enclosures for hamsters and gerbils should be easy to clean, lightweight, and should have a plastic bottom and sides deep enough to contain bedding. Additionally, these enclosures should be well ventilated. Large door/doors should be provided for removal of the animal from the cage. Small plastic heavy dishes are recommended for food and sipper bottles for water. Water deprivation is a common problem in pet rodents and should be avoided by inspection of sipper bottles or water containers daily for function, water level, and cleanliness. Hamsters and gerbils should be kept in a clean environment to avoid disease problems. Food and water containers should be cleaned with dilute soap weekly and washed free of debris on a daily basis. Enclosures should be thoroughly cleaned on a weekly basis, by removing all bedding and cleaning the enclosure with soap and hot water. Solid plastic cages should be cleaned more regularly as they have less ventilation. Inspection and removal of solid and liquid wastes should occur on a daily basis. As a general rule, if one can smell the enclosure, it must be cleaned. Many rodents suffer mortality and morbidity from ammonia build-up and improperly cleaned or maintained enclosures.