In order for the food scientist to understand the characteristics of fresh meat and to appreciate its potential for incorporation into processed meat, he must have a fundamental knowledge of muscle. The casual observation is often made that certain muscles are redder or darker than others. The level of organization, be it red and white muscle, or red and white fibers, is a situation of terminology and recognition that exists, and must be understood. Enormous differences in composition and morphology exist between red and white muscle. Red muscle has a greater concentration of the pigment myoglobin, is generally lower in soluble protein content, lower in glycogen, and higher in lipid than white muscle. Red fibers are smaller in size than white fibers, are better supplied with capillaries, and contain more mitochondria. White fibers are equipped better for glycolytic metabolism than are red fibers, which are designed for oxidative metabolism. Isolated sarcoplasmic reticulum fragments from white muscle have a greater in vitro calcium-binding ability than those from red muscle and the myosin from red muscle has a lower ATPase activity than that from white muscle. The physiological response of red and white fibers corresponds well with what has been stated about their composition and biochemical properties. Stimulation of a white fiber produces a twitch response; a twitch response is also elicited in red fibers, but it gives a slower contraction which covers a longer period of time than that from a white fiber. Histochemical techniques have produced many beautiful and useful pictures of fiber type distribution. Red fibers are high in oxidative enzymes, such as SDH, but low in glycolytic enzymes, such as phosphorylase and also low in ATPase; the opposite situation exists in white fibers. Intermediate fibers are well recognized. The fiber types differentiate clearly just before or after birth of the animal. This is demonstrated by histochemical, biochemical, and physiological techniques. Because the control of differentiation is such an important biological problem, experiments dealing with cross-innervation and the trophic influence are presented. Finally, it is pointed out that fiber type composition has been shown to be an important consideration in the conversion of pig muscle to food for human consumption. Suggested areas of research need are noted.