Physiology and genetics of early maturity in cereals are the subject of practical as well as scientific interest for agronomist and plant breeders. Thorough understanding of the true nature of such a complex character requires physiological and genetical knowledge about the internal factors, which are closely bound up with and react to some particular external or environmental factors. From the practical point of view, experiments should be conducted under controlled conditions, especially the day length and temperature, so that the genotypic differences pertaining to these factors may be discerned. Takahashi and Yasuda (1958, 1970) maintained that at least three physiological factors were responsible for determining earliness in barley, namely, (1) spring and winter habit of growth or vernalization requirement, (2) photoperiodic response or sensitivity to short-day, and 3n earliness factor in a narrow sense or minimal vegetative growth. The same situations were true in common wheat also (Yasuda and Shimoyama, 1965). In this report, physiology and genetics of internal factors and their relations to the time of heading in the field will be presented with some problems concerning differences in mechanism of early maturity between barley and wheat.