The rotation of our planet results in regular changes in environmental cues such as daylength and temperature, and organisms have evolved a molecular oscillator that allows them to anticipate these changes and adapt their development accordingly. In many plants, the transition from vegetative to reproductive growth is controlled by photoperiod, which synchronises flowering with favourable seasons of the year. Here, we describe the notable progress that has been made in identifying the molecular mechanisms that measure daylength and control of flowering time in Arabidopsis , a long day (LD) plant, and in rice, a short day (SD) plant. Although the components of the Arabidopsis regulatory network seem to be conserved in other species, the difference in the function of particular genes may contribute to the reverse response to daylength observed between LD and SD plants. We also highlight the recent advances in understanding the regulatory mechanisms that underlie other developmental transitions controlled by photoperiod, including tuberisation and the onset of dormancy in the buds of perennial plants. © 2006 Taylor & Francis.