The goal of the study was to assess the spatial differentiation and temporal variability of thermal resources in Poland in the period 1951-2010 in response to ongoing changes of thermal conditions in Europe. The research was based on daily temperature values. Thermal resources have been defined by Growing Degree Days index (GDD) calculated for three air temperature thresholds: 0˚C, 5˚C and 10˚C. The detailed analysis of the spatial differentiation of thermal resources was followed by the long-term variability and trends evaluation. The analysis was conducted for the whole country and, because perennial crops are particularly vulnerable to the variability of thermal conditions, the largest fruit-growing regions in Poland in terms of growing area size and the size of fruit harvests (apples, plums, cherries). The obtained results confirm the increase of thermal resources in Poland and indicate the lengthening of the growing season as the main cause.