An organism’s response to the physical environment is determined by its immediate abiotic surroundings, which can have significant spatio-temporal variability. Multi-year examination of seawater temperatures on coral reef communities in the lagoon of Moorea, French Polynesia, and the fringing reefs of Nanwan Bay, Taiwan, revealed a strong pattern of diurnal thermal fluctuations of up to 4.5 ºC and 9 ºC, respectively. Repeated experiments were carried out in Moorea in April and May of 2006 and 2007 in order to determine the effects of large diurnal fluctuations in temperature on the physiology of the scleractinian coral host and its symbiotic dinoflagellates in the genus Symbiodinium. In 2006, exposure of the common reef corals Pocillopora meandrina and Porites rus to extreme fluctuations in temperature, ~6 ºC every 12 h, resulted in significant declines in maximum dark-adapted quantum yield (FV/FM) and Symbiodinium density when compared to a steady ambient treatment. In 2007, exposure of Pocillopora meandrina and Porites rus to diurnal fluctuations in temperature (26 – 30 ºC), compared to three steady temperature treatments, resulted in a decrease in the Symbiodinium density of corals in response to treatment. Importantly, symbiont density showed significantly greater reductions in the fluctuating treatment than in corals exposed to the steady treatments. However, a measure of holobiont performance - skeletal growth - remained stable in all treatments. The effects of thermal variability also were studied on the reef corals in Taiwan in 2007. Here, both adults and larvae of the common brooding corals Pocillopora damicornis and Seriatopora hystrix were used to examine ontogenetic variability in the response of reef corals to fluctuating temperatures. In addition, a time series examination of the physiology of freshly-collected P. damicornis larvae from sequential release days was conducted to test the hypothesis that coral larvae differ physiologically with time spent in the maternal polyp. The results of this time series study revealed that three parameters - FV/FM, Symbiodinium density, and larval size - differed with day of release. The largest differences were seen in the variation in Symbiodinium density and larval size. Following exposure to steady temperatures, FV/FM of coral larvae were significantly reduced (~53% on average) compared to the FV/FM of adults. However, in the fluctuating treatment, FV/FM was only reduced ~36% on average when compared to adult values. Together these results show that large fluctuations in the thermal environment can have detrimental effects on coral physiology depending on the duration of extreme fluctuations and the amplitude of the temperature signals. Nonetheless, corals respond in an extremely flexible manner revealing that they are able to acclimatize to thermally heterogeneous environments.