Seed development involves the formation of the embryo, endosperm, and seed coat. Endosperm development is not only crucial for embryo development and seed maturation (i.e., plant reproductive growth) but also affects grain yield and crop quality, especially in cereal crops such as rice, wheat, and maize. Elucidating the relevant regulatory mechanisms and genetic networks is important from a basic research perspective and may have implications for agricultural improvements. Studies have revealed the tightly regulated cytological development of the endosperm as well as the associated genetic and epigenetic controls. This chapter summarizes the cytological basis and genetic networks of endosperm initiation, cellularization, aleurone layer specification, accumulation of storage materials, and interactions between the embryo and endosperm.