Milk contains numerous important nutrients such as fat, protein, lactose, mineral salts, and vitamins. Milk composition is economically important to milk producers and processors and nutritionally important to consumers. Variations in milk composition arise from differences in relative rates of synthesis and secretion of milk components by the mammary gland. The processes involved for lactose, protein, and fat synthesis and secretion are independent but regulated through nutrient or substrate availability and hormonal control of nutrient utilization. Thus, genetics, which mediates hormonal effects, and diet, which regulates nutrient availability, are the major factors affecting milk composition.