Summary 1 The paper reviews the literature on seed dormancy, with special regard to incon­ sistencies in terms and definitions used. It presents a concept of seed dormancy in which physiology and ecology are integrated. Its aim is to increase the understanding of seed dormancy and germination, and to help defining ecological research questions. 2 It is claimed that seed dormancy should not be identified with the absence of germination. Seed dormancy should rather be defined as a characteristic, the degree of which determines the range of conditions in which a seed is able to germinate. Dormancy varies on a continuous scale, which is visualized by continuous changes in the range of conditions suitable for germination. If the conditions required by the seed are met by its environment, the seed will germinate. 3 The concept of dormancy that is described in the paper is partly based on a physiological model for the regulation of dormancy and the stimulation of germi­ nation. In this model dormancy is related to the amount of a hypothetical phy­ tochrome receptor in the seed. 4 It is argued that the process of dormancy release should be clearly distinguished from the germination process itself. It is stated that as yet only temperature has been shown to alter the degree of dormancy in seeds. Factors like light and nitrate are often indispensable for germination, but only by promoting the germination process itself, not by mitigating the requirements for germination. 5 It is suggested that seed dormancy prevents germination when conditions are favour­ able for germination, at a time of the year when it can be expected that the plant originating from the seed will not survive and produce offspring. 6 It is concluded that dormancy should not be regarded as inactivity of seeds. At any degree of dormancy, seeds continuously react to their environment by adjusting their level of dormancy to the changing environment.