Inflorescence structure directly affects the yield of grain crops. Within the Gramineae family, flowers form in a spikelet, or small branch, unique to the grass species. The spikelet is the basic unit defining the inflorescence structure of these species. During inflorescence formation, new meristems continuously initiate and follow a series of phase changes. Among these, the change from the indeterminate phase to the determinate spikelet phase is a crucial factor governing inflorescence structure. Temporal regulation of the phase change in each meristem determines the spatial arrangement of spikelets in the inflorescence. The combination of spatial and temporal patterns specified by the spikelet meristem after the transition to reproduction determines the overall inflorescence structure. Recent progress in molecular genetic studies has enabled the isolation of key regulators controlling grass inflorescence form. Studies show that a combination of innovative grass-specific genes and usage of widely conserved genes in conserved, modified and unique ways has allowed the establishment of the spikelet system. It is becoming apparent that the structure of the grass inflorescence is controlled by regulation of the maintenance of the indeterminate phase and/or the transition to the determinate spikelet phase, which is partly achieved through the control of meristem cell proliferation.