Seasonal variability in environmental parameters such as day length regulates many aspects of plant development. The transition from vegetative growth to flowering in Arabidopsis is regulated by seasonal changes in day length through a genetically defined molecular cascade known as the photoperiod pathway. Recent advances were made in understanding the tissues in which different components of the photoperiod pathway act to regulate floral induction. These studies highlighted the key role of the FT protein, which is produced in the leaves in response to inductive day lengths and traffics through the phloem to initiate flowering at the shoot apex. Unveiling the cellular and molecular details of this systemic signaling process will be required for a complete understanding of flowering regulation and other photoperiodic processes.