Pollen grain, also referred to as a male gametophyte, is a male reproductive structure that produces male gametes (also called sperm cells) and plays important roles in the delivery of the male gametes into female gametophytes during pollination and fertilization. Pollen development is initiated from the microspore generated by meiosis at the middle stages of anther development. The microspore undergoes an asymmetric pollen mitosis I (PMI) to form a bicellular pollen grain that consists of a vegetative cell and a generative cell. The generative cell further has a symmetric pollen mitosis II (PMII) to generate two sperm cells and form a tricellular pollen grain. As a result, the pollen grain contains two smaller sperm cells that are enclosed in a larger vegetative cell. The tricellular pollen grain is further dehydrated to develop into a mature pollen grain ready for pollination and fertilization. In this chapter, the molecular genetics of pollen development will be discussed, including the genes and their molecular mechanisms that are involved in pollen mitosis, cell specification, and pollen wall formation.