Extrusion cooking was used to manufacture pet foods during the early 1950s. Today, extrusion is used to make semi-moist and dry expanded pet foods, aquatic food, milk foods, and foods for laboratory animals. Extrusion offers a way to cook, shape, and pasteurize these products. It permits better utilization of available cereal grains, as well as vegetable and animal proteins, in manufacturing cost-effective and nutritionally balanced diets with unique and improved characteristics. Since this article focuses mainly on products for human consumption, extrusion of pet foods and animal feed will not be discussed in the sections that follow. Even though the applications of extrusion technology in food processing and manufacturing have broadened over the last two decades, the science and fundamental understanding of the process have always lagged behind the technology. The complex, multi-component, irreversibly reactive nature of food materials, combined with complex screw and die geometries, make complete deterministic analyses of extrusion processes extremely challenging. Issues related to the predictability and control of the flavor and texture of extruded products still need more research and development. In the area of nutraceuticals (health-promoting compounds found mainly in plants), extrusion might offer a way to improve their extractability, enhance their bioavailability, and deliver them effectively.