In this chapter, we have highlighted recent advances in our understanding of ABA and GA metabolism during seed development and germination in Arabidopsis. Identification of the majority of ABA and GA metabolism genes in this model species (Hedden et al., 2002; Nambara and Marion-Poll, 2005; Marion-Poll and Leung, 2006) has enabled researchers to study how the amounts of these two hormones in seeds are regulated in specific developmental phases and under defined environmental conditions. To fully understand the regulation of ABA and GA levels, it will be necessary to identify additional uncharacterized ABA and GA metabolism enzymes, as these two hormones are likely to be deactivated via multiple pathways. Evidence has now been provided that well-studied developmental regulators, such as FUS3 and LEC2, play direct or indirect roles in modulating hormone levels in developing seeds (Curaba et al., 2004; Gazzarrini et al., 2004). Identification of additional key regulators of ABA and GA levels will increase our knowledge on how cellular concentrations of these antagonistic hormones are balanced during seed development and germination. The application of large-scale expression analysis to seed biology (Ogawa et al., 2003; Nakabayashi et al., 2005) has been a powerful tool to facilitate our understanding of the overall regulation of hormone metabolism pathways. Such genome-wide analyses in combination with molecular genetic studies will allow us to uncover new molecular links among different hormones as well as connections among hormonal and developmental or environmental regulators during seed development and germination.