The evolution of thermal resistance and acclimation is reviewed at the population level using populations and isofemale lines of Drosophila buzzatii and D. melanogaster originating from different climatic regions. In general, ample genetic variation for thermal resistance was found within and among populations. A rough correlation between the climate of origin and thermal resistance was apparent. Acclimation at a non-lethal temperature led to a significant increase in survival after heat shock, and recurrent acclimation events generally increased survival even further. Acclimation effects lasted over several days, but this effect decreased gradually with time since acclimation. Protein studies showed that the concentration of Hsp70 in adult flies is greatly increased by acclimation and thereafter gradually decreases with time. For populations with relatively high survival at one life stage, survival often was low at other life stages. Furthermore, selection on different life stages showed that a selection response in one life stage did not necessarily result in a correlated response in another. These observations indicate that different mechanisms or genes at least in part are responsible for or are expressed at different developmental stages. Selection for increased resistance was successful despite low heritabilities for the trait. Survival and fertility were compared between acclimated and non-acclimated flies, and a cost of expressing the "heat shock response" was identified in that increased survival after acclimation was accompanied by reduced fertility. The relative costs increased under nutritional stress. Metabolic rate was genetically variable but did not correlate with temperature resistance. The more resistant lines, however, often had shorter developmental time. Inbreeding reduced thermal stress tolerance of adult flies, but it did not reduce tolerance of embryos that possibly are exposed to strong natural selection for thermal stress resistance. In general, inbreeding may reduce stress resistance, and thus multiple stressful events may account for increased inbreeding depression in harsh environments.