The account presented in the previous two chapters has established that the differentiation of the diploid microsporocyte proceeds in several discrete stages to produce a two- or three-celled haploid pollen grain with a complex structural and functional organization. Included in the progression of events in this multistep process are meiotic division and the evolution of cells endowed with different characteristics, form, and function. The existence of a rightly controlled series of cytological and biochemical changes associated with pollen development and maturation underlies a coordinated gene expression pattern, involving genes from both gametophytic and sporophytic generations. Although we do not yet fully understand how the genetic program operates during pollen ontogeny, we have some notion of its machinery and how the machinery might be used. This understanding has come from three independent approaches – genetic, molecular–biochemical, and gene cloning – that have been pursued from time to time during the past 50 years or so. At the genetic level, examination of the segregation of pollen characters and of spontaneous and induced mutations has indicated that a very large set of genes is expressed during male gametogenesis. Molecular–biochemical studies have included analyses of the accumulation and synthesis of nucleic acids and proteins during anther and pollen development. The main thrust of the research in recent years has been on the isolation and characterization of anther- and pollen-specific genes by differential screening of cDNA libraries from whole anthers and pollen grains.