Dormancy is an important property of seed potatoes. Seed tubers planted too soon after their harvest do not produce plants because of dormancy, or produce low yields because of poor growth vigour. Potato tubers from the same cultivar vary in their duration of dormancy. The first aim of the research reported in this thesis was to predict the end of seed tuber dormancy, and to explain the variation in the duration of dormancy. The second aim was to investigate ways of curtailing the dormancy and of advancing the growth vigour of seed tubers. Tubers harvested when immature were used in the study.After haulm pulling, the length of the axis and the number of leaf primordia of the tuber buds did not change during dormancy, and therefore these characteristics cannot be used to quantify dormancy. The longer tubers had been kept in storage, the stronger their sprouting response to a dormancy-breaking treatment with a growth-stimulating substance. This indicates that the intensity of dormancy is not constant during the dormancy period. It is concluded that this response might be used to predict the end of dormancy, but more research is necessary to develop such an application.The variation in duration of dormancy within a seed tuber lot depended on the cultivar and was large. In cv. Diamant, this variation was found to be related to variation in tuber weight, date of tuber initiation and the position of the tuber on the plant during its growth. The effect of growth conditions such as nitrogen, temperature, light intensity and photoperiod on dormancy of progeny tubers was minor, but the effect of storage conditions immediately after the harvest was major.Dormancy was greatly shortened by a foliar spray with gibberellic acid applied shortly before haulm killing, and by storage regimes with low temperatures (2 °C) and even much more so by those with high temperatures (28 °C). The growth vigour of the tubers was greatly advanced by both a foliar spray with gibberellic acid and by storage at 28 °C. The effects varied from a single week up to several (>3) months depending on the duration of dormancy of the cultivar, whether one or both treatments were used, and on whether a storage regime with a low or high temperature was opted for. The treatments offer very good prospects for improving the performance of seed potatoes that have to be planted soon after harvest.