This chapter describes the contributions that Gibberellin (GA) and abscisic acid (ABA) mutants have made to the understanding of the hormonal control of seed development, dormancy, and germination. Phenotypic aberrations are not known for seeds of auxin, cytokinin, and ethylene response mutants. With respect to the GA mutants, most attention has been paid to the various GA responsive dwarfs in garden pea, sweet pea, rice, wheat, maize, and bean. ABA-deficient mutants have been recognized by their increased tendency to wilt on a mild exposure to water stress or by a reduced seed dormancy. The free ABA content in seeds is high during development and is relatively low or even absent at maturity. Primary dormancy develops in seeds when they are on the mother plant. The study of hormone mutants has made a strong contribution to our understanding of the control of dormancy and germination.