Development of wheat (Triticum spp.) is primary driven by temperature, but is also affected by other factors such as vernalization and photoperiod. Crop growth and development are often described in terms of calendar days. However, determining the development in terms of thermal time or physiological time is more accurate because it is an accumulation of the caloric energy needed for the occurrence of phenological stages. The objectives of this study were: (i) to determine the base temperature for key phenological stages of different winter wheat cultivars and (ii) to develop a phenological model using the base temperature for predicting the duration in terms of thermal time for different phenological stages. Eight wheat cultivars were selected according to their vernalization period to determine the base temperature for three critical developmental phases, i.e., planting to heading, heading to harvest and planting to harvest. For each cultivar, the base temperature for each critical period was estimated as well as the duration of the three key phenological stages in terms of thermal time for three locations in Georgia from 1999 to 2010. The base temperatures and the growing degrees varied widely depending both on the developmental stage and the cultivar. The estimated base temperatures for the eight wheat cultivars ranged from 3.1 to 8.1oC, 10.6 to 18.4oC and 1.6 to 8.4oC, for planting to heading, heading to harvest maturity and planting to harvest maturity. Also, the duration in Growing Degree Days (GDD) was determined for each season and cultivar. When 0oC was used as the base temperature, the GDD between cultivars varied from 1675-1844, 1017-1239 and 2827-2936oC from planting to heading, heading to harvest maturity and planting to harvest maturity, respectively. The results from this study provided specific base temperatures for each developmental stage for each individual cultivar and, therefore, provided a more accurate estimation of GDD. The variation in base temperature and GDD accumulation is probably a selective advantage for winter wheat. Clearly more work is required to estimate the base temperatures and duration for others phenological stages and further evaluation is required for additional cultivars and a wider range of environments.