Plants are an important group of organisms which are capable of auxotrophic growth and therefore have a vital function to maintain life on earth. Although a number of plant species reproduce asexually, most plant species use specialised structures, flowers, for sexual reproduction. Compared with animal systems surprisingly little is known about the molecular processes governing organogenesis. However during the last five years a number of regulatory genes have been identified, isolated and characterized which are controlling the onset of developmental programmes leading to the determination and development of the various floral organs. The function of these genes was studied using genetic and molecular genetic techniques. Three classes of genes can be discriminated which are determining the developmental switches from vegetative meristem (class I), via floral meristem (class II) into the various organ primordia (class III). The genes involved encode transcription factors which function in a complex network of regulatory proteins. The final fates of the floral organ primordia are determined by the so-called ABCD genes. The activity of those genes can be summarised in the ABCD model which provides a simplified way to describe how the identity of each floral organ is determined.