Mastitis is an inflammatory condition of the udder, regardless of the cause, characterized by physical, chemical and biological microchanges in the milk, and pathological changes in the glandular tissue of the udder. It can be clinical or sub-clinical. The latter is defined by qualitative and quantitative changes in milk, usually without clinical signs or macroscopic changes in the milk. Subclinical mastitis is 3 to 4 times more common than clinical mastitis and the cause of great losses in the dairy industry (Jasper, McDonald, Mochrie, Philipot, Farmworth and Spencer, 1982).This work is divided into seven parts and reviews the current knowledge of mastitis in small ruminants. First, a review of the anatomical and physiological characteristics of lactation will be given. In this part, we will see the differences in the type of milk secretion between ewes and goats.