The ABC-model explains the genetic basis for the specification of floral organ identities, stating that A-function is required for the sepal, (A + B)-functions are necessary for the petal, (B + C)-functions are needed for the stamen, and C-function is essential for the carpel. Molecular cloning revealed that all ABC-genes encode transcriptional regulators and most are MADS-box genes. This chapter focuses on molecular genetic studies that have identified numerous genes that affect A-, B-, and C-functions by regulating specific genes that are required for A-, B-, or C-function. These genes act at both the transcriptional and posttranscriptional levels and encode a variety of regulatory proteins. Discoveries and analyses of these genes provide further understanding of the molecular mechanisms underlying floral development and lay a foundation for greater advances in this fertile field of plant development. Future studies using genomic and evolutionary approaches promise to reveal an expanding knowledge of the floral molecular machinery.