Through genetic selection, improvements in nutrition and management, and advances in milking technology, the mammary gland of the dairy cow yields far more milk nutrients and volume than the calf can consume. Selection of cows for greater production, and the stresses associated with nutrition, reproduction, and the environment may affect milk yield as well as composition. The quantity and quality of milk are highly dependent upon the amount of mammary tissue available to produce milk, secretory cell efficiency in synthesizing milk components, and the availability of suitable nutrients from which the cow manufactures milk. The purpose of this chapter is to provide a basic knowledge of the factors that affect milk yield and composition, which should aid in the development of procedures to efficiently harvest large volumes of a high-quality product. Before such factors can be considered, a basic understanding of mammary gland anatomy, milk storage, and the physiology of milk component biosynthesis and let-down is presented.