The processing of cereals and other crops by extrusion involves various phenomena, and despite their diversity, it can be conveniently described by the succession of two main mechanisms: (1) the transition from a divided solid medium (the powdery or granular material, the flour, etc.) into a continuous viscous matrix (the melt, the dough, etc.) due to mechanical and heat actions, in the screw channels (2) a liquid–solid transition, often accompanied by expansion of the viscous continuous matrix towards a solid cellular structure, as a result of the growth of gas bubbles due to water vaporization after a sudden pressure drop at die outlet. These mechanisms can be described by integrating knowledge from rheological models, that reflect material changes related to the behavior of raw materials submitted to high shear and temperature (in relation to Chapters 4 and 545). These simple models may be inspired by other scientific domains, chemical engineering, and powder technology, but they possess operational interest for extrusion. They can be applied to a wide domain of recipes, provided that the data on product rheological properties are available. Finally, it is suggested that such an approach can help food engineers to design cereal food products and predict their texture properties. In this chapter, we will describe the two main mechanisms with emphasis on the experimental methods that enlighten them. Examples will be given to illustrate how they can be addressed by rheological models and can, in turn, be helpful for product development.