This article is a modified paper, presented to the 10th International Botanical Congress, Edinburgh 1964. Vernalization is primarily discussed from an ecophysiological viewpoint, serving as a basis for understanding the mechanism. Eco-physiological observations point to the part which the substratum plays and also demonstrate the great variability among the requirements for vernalization, both regarding the low temperature treatment in itself and its relation to other factors like day length. The existence of a genetical basis for vernalization shows that DNA has something to do with it, or: that the first step in the vernalization process is a change in the regulation of gene activity. The loci of vernalization are considered to be mitotic cells, so that an important part of the mechanism of vernalization is the influence of low temperature on mitotic cells. This involves a study of nucleic acids. Gibberellins play an important part in vernalization and it may well be that this part will appear to be greater, when more knowledge will be gathered about what can be briefly indicated as: the biosynthesis of gibberellins.