Plant development is governed by intrinsic and environmen- tal factors that regulate the identity and activity of meristems, organized tissues of pluripotent "stem" cells, that together de- termine plant form and architecture. However, little is known about how these factors act at the molecular leve1 to affect meristem identity and function. Genetic studies in Arabidop- sis and other plant species such as snapdragon, petunia, and maize have revealed a hierarchy of regulatory genes that func- tion together to promote the formation of the floral meristem and to regulate floral organogenesis. Mutations in these genes result in dramatic defects in flower development that can af- fect both meristem identity and organ development. Table 1 lists the Arabidopsis genes that are known to control meristem and organ identity and their snapdragon equivalents. One class of regulatory genes, the homeotic organ identity genes, have provided important insights into the genetic and molecular mechanisms that govern floral organ identity and development (Bowman et al., 1991; Coen and Meyerowitz, 1991; Coen and Carpenter, 1993, this issue; van der Krol and Chua, 1993, this issue). In Arabidopsis, these genes include