The timing of floral transition has a direct impact on reproductive success. One of the most important environmental factors that affect the transition is the change in day length (photoperiod). Classical experiments imply that plants monitor photoperiods in the leaf, and transmit that information coded within an elusive signal dubbed florigen to the apex to reprogram development. Recent advances in Arabidopsis research indicate that the core of the day-length measurement mechanism lies in the circadian regulation of CONSTANS (CO) expression and the subsequent photoperiodic induction of the expression of FLOWERING LOCUS T (FT) gene, which might encode a major component of florigen. In this review, we introduce current perspectives on how, when and where the floral signal is generated.