The number and placement of lateral roots on the primary root is not predetermined, but can be drastically affected by the availability of water and nutrients in the soil. To respond to local and often transient conditions, the plant must be able to perceive environmental cues and respond by initiating lateral roots in appropriate positions. This involves recruiting small numbers of mature cells to resume dividing and enter a program of postembryonic organogenesis. Through the use of new mutants and molecular markers of cell differentiation, the molecular mechanisms involved in lateral root initiation and development are beginning to be understood.