Abstract: Criteria used to measure performance, environmental factors that influence performance and estimates of heritability are needed to estimate genetic differences. Published heritability estimates of various measures of performance in the horse are summarized. The average heritability estimates of pulling ability and cutting ability are .25 and .04, respectively. Heritability estimates are .18, .19 and .17 for log of earnings from jumping, 3-day event and dressage performance, respectively. Heritability estimates of performance rates, log of earnings, earnings, handicap weight, best handicap weight, time and best time for the Thoroughbred are .55, .49, .09, .49, .33, .15 and .23, respectively. Heritability estimates of log of earnings, earnings, time and best time for the trotter are .41, .20, .32, and .25, respectively. The heritability estimate of best time for the pacer is .23. The effectiveness of selection will depend on which performance trait is to be improved.