Photoperiodism is the response to the length of the day that enables living organisms to adapt to seasonal changes in their environment as well as latitudinal variation. As such, it is one of the most significant andcomplex aspects of the interaction between plants and their environment and is a major factor controlling their growth and development. As the new and powerful technologies of molecular genetics are brought to bear on photoperiodism, it becomes particularly important to place new work in the context of the considerable amount of physiological information which already exists on the