Plant root system architecture (RSA), the spatial configuration of a root system in the soil, is critical for water and nutrient acquisition. Rice generates a root system consisting of seminal, lateral, and crown/adventitious roots, whose growth and development are regulated by plant hormones and can be fine-tuned to meet adverse environmental conditions. A broad range of studies have been carried out in the past to decipher the underlying molecular and genetic mechanisms. This chapter summarizes original and more recent findings on genes and molecules identified so far to be involved in rice root development. It also discusses the cellular organization and function of rice roots, as well as the responses of RSA to the availability of water and nutrients.