Heat stress can be defined as the physiological stress that animals experience when the environmental temperature is above their thermoneutral zone. Dairy cattle begin to experience heat stress around 20°C. During heat stress the animal may have reduced milk yield, feed intake, and reproductive performance. However, when accessing heat stress external factors affect the threshold temperature such as radiant energy, relative humidity, and wind speed. To account for some of these variables, a metric called the Temperature-Humidity Index (THI) is often used. When an animal is more resistant to heat stress, it is considered to be more heat tolerant. Heat tolerance is important in dairy cattle because their bodies produce a large amount of internal heat. Dairy producers select for greater milk production, requiring cattle to increase DMI, indirectly increasing body heat production. When producers make breeding selections based on milk production, heat tolerance decreases. A negative correlation(R = −0.3) exists between milk production and heat tolerance (Ravagnolo and Misztal, 2000). It may be possible to improve both because there is a low correlation between milk production and heat tolerance traits. Milk yield data were collected from 366,000 cows and compared with weather data for an 11-year period. The cows were divided into heat tolerant and heat susceptible groups based on their decrease in milk production while experiencing heat stress. Genomic information was analyzed to predict genomic estimated breeding values for heat tolerance. Two groups of 24 cows that were genetically identified as the most heat susceptible or heat tolerant were exposed to high heat conditions for 4 d. Cows that were genetically typed as heat tolerant displayed a lower reduction in milk production and DMI (Garner, 2016). As genomics continue to improve and heat tolerance is better understood, selection for more heat tolerant dairy cattle may be possible in the future.