Flowers are unique to angiosperms. The origin and diversification of flowers have contributed to the great success of angiosperms in the terrestrial ecosystems. While a typical flower is composed of four types of organs: sepals, petals, stamens, and carpels, flowers in some angiosperms have evolved various other organ types, such as petaloid sepals, tepals, labella, lemmas, paleas, lodicules, and staminodes, which have greatly contributed to floral diversity. The elucidation of how different types of floral organs are specified is important for understanding the underlying mechanisms responsible for the origin and diversification of flowers. Since the proposal of the ABC model of flower development, the processes and mechanisms of floral organ identity determination have been investigated extensively in two pioneering model systems, Arabidopsis thaliana and Antirrhinum majus, as well as quite a few species from phylogenetically distant lineages. In the past 30 years, impressive progress has been made in identifying the patterns, mechanisms, and evolution of floral organ identity determination. In this chapter, we first briefly summarize the patterns of floral organ initiation and specification, and then provide a detailed overview of how floral organs of angiosperms and reproductive structures of gymnosperms are specified. Finally, we present current understanding of the evolutionary origins of the floral organ identity determination programs. We suggest that future research should focus on the regulatory evolution of floral MADS-box genes, functional and evolutionary analyses of non-MADS-box floral organ identity genes, and development of new model systems in basalmost angiosperms and gymnosperms.