Cooperation between rider and horse is of major importance in equitation. A balanced team of horse and rider improves (sport) performances and welfare aspects by decreasing stress, frustration, risks of injuries, and accidents. Important features affecting the cooperation are the physical skills, knowledge, and personality of the rider on one hand and the temperament, experience, and physical abilities of the horse on the other. A study with 16 riders and 16 warm-blood riding horses tested the effect of personality of riders and temperament of horses on cooperation between riders and horses. More emotionally reactive horses showed more evasive behavior during riding. Riders preferred to ride those horses who were assessed by the riders as being attentive to the rider's aid. The frequency of evasive behaviors during riding--as assessed by riders, in contrast to the assessments made by an external judge--influenced the cooperation between rider and horse. On average, a rider's personality did not affect the cooperation between rider and horse; however, it is suggested that a rider's personality does affect the cooperation with more emotionally reactive horses.