In the reproductive biology of angiosperms, the microspore and pollen grain represent paradigms for studies on cell differentiation. As described in Chapter 4, an important reason for this is that by a program of gene expression regulated in space and time, the unicellular microspore matures into the pollen grain and embarks on a pathway leading to terminal differentiation by the production of two cells with divergent developmental potential. A second reason is that the induction of embryogenic divisions in microspores or pollen grains of certain plants by tissue culture methods makes it possible to gain insight into mechanisms involved in the deflection of a typical gametophytic program into an atypical sporophytic pathway. This chapter will focus on this alternative developmental pathway of microspores and pollen grains, which gives rise to embryoids and plantlets with the haploid or gametic number of chromosomes. This phenomenon is known as androgenesis, haploid embryogenesis, or pollen embryogenesis, but the last-mentioned term is preferred for use in this book. Irrespective of how the morphological and cytological changes converge to cause gametophytic or sporophytic types of growth in pollen grains, it seems certain that the same genetic blueprint is utilized to meet the informational demands of these transformation episodes.