Regulation of flowering is one of the most important processes of plants since it is closely related to the success of reproduction. Flowering time is controlled by a number of environmental factors such as day length, temperature, and water supply. Among them, the photoperiod is a key regulator of flowering and has been studied for many years (Figure 1). However, molecular genetic study of the photoperiodic regulation of flowering began only about 10 years ago. Arabidop- sis thaliana, a long-day plant, has been extensively used to study the photoperi- odic regulation of flowering, and a large number of genes involved in flowering time determination have been isolated and characterized. As a result we now understand the genes involved in day length control of flowering in a long-day plant relatively well. However, our knowledge on genes involved in flowering in short-day plants has been limited mainly because many model short-day plants have not been amenable to molecular genetic analysis until recently.