Development of wheat (Triticum spp.) is primary driven by temperature, but affected by other factors such as vernalization and photoperiod. Crop growth and development is often described in terms of calendar days. However, determining the development in terms of physiological time (thermal time) is more accurate since is independent of environment or year. The goals of this study were: (i) to determine the base temperature of key phenological stages for winter wheat and (ii) to establish their duration in terms of thermal time. In this study six wheat cultivars were selected according to their vernalization requirements to determine the base temperature for three different periods, from planting to heading, heading to harvest and from planting to harvest. Data were obtained from three locations in Georgia, including Griffin (33°26’N, -84°28’W, elev. 285 m), Plains (32°05’N, -84°37’O, elev. 158 m), and Tifton (31°49’N, -83°53’W, elev. 285 m) and from 1999 until 2010.The base temperatures for growing degree varied widely depending both on the development stage of the crop and the cultivar. From planting to heading the estimated base temperatures for the six wheat cultivars ranged from 3.01 to 8.12 °C, from heading to harvest maturity it ranged from 10.62 to 18.38°C and from planting to harvest maturity it ranged from 1.6 to 8.41 °C.  Also, the duration in GDD (°C) was determined for each season and cultivar. These results show that there is a need to develop an appropriate model that uses differential base temperatures as a function of crop stage.