Literature on the germina-tion of winter annuals is reviewed. It suggests that the timing of germination in various species is controlled by dormancy, changes in the temperature response of the seed with aging, temperature, soil moisture, or some combination of these. This paper attempts to account for the precise timing of germination in Aira praecox and and Teesdalia nudicaulis. Experiments in controlled temperatures show that in both species the seeds are at first capable of rapid germination only at low temperatures, but with increasing length of storage they can give rapid germination at higher and higher temperatures. By comparing these results with records of germination and temperature in a natural habitat, it is shown that in the field the germination of Aira is normally controlled by an interaction of soil moisture, temperature, and the changing temperature response of the seed. In Teesdalia, because of faster change in temperature response, soil moisture plays a relatively more important role.