A variety of endocrine and neoplastic conditions are commonly observed in rodents in clinical practice. Only some of these diseases are reported in the literature and the successful diagnosis and treatment of such diseases are is even more rarely described. There are many reasons for this, but the problems are principally: the small size of most of these species; the difficulty in obtaining adequate or repeated samples; interpreting test outcomes when rodent responses to an established cat or dog test can be highly variable; and the paucity of information on reference ranges. This chapter will list the main endocrine and neoplastic diseases likely to be encountered, give a guide to aetiology, clinical signs and the differential diagnoses and, where possible, describe treatment or management regimes. An overview of the diseases discussed for each species is given in table form at the end of each section.