Plants are sessile organisms and must respond to changes in environmental conditions. Flowering time is a key developmental switch that is affected by both day length and temperature. Environmental cues are sensed by the leaves while the responses occur at the apex, requiring long-range communication within the plant. For many years it has been known that leaves exposed to light can trigger the floral transition of a darkened shoot, and grafting experiments demonstrated that the floral stimulus travels long distances. This mobile signal was later termed "florigen," but its nature has been unclear. The gene FLOWERING LOCUS T (FT) is a major output of both the photoperiod and the vernalization pathways controlling the floral transition. FT protein acts at the shoot apex of the plant in concert with a transcription factor, FLOWERING LOCUS D (FD). A fundamental question is how FT transcription in the leaves leads to active FT protein at the apex. We have uncoupled FT protein movement from its biological function to show that FT protein is the mobile signal that travels from the leaves to the apex. To our knowledge, FT is the only known protein that serves as a long-range developmental signal in plants.