Collisions with obstacles, such as trees, rocks, and other people, are a common occurrence in the sports of skiing and snowboarding. Once an obstacle becomes visible, whether or not the skier has time to avoid it is largely determined by that skier's reaction time (RT)—the time it takes to detect and identify the obstacle, make a decision about how to respond, and initiate that response. Stopping and turning RTs were measured in ten expert skiers and four expert snowboarders at Mammoth Mountain, California. Participants were told to search for a sign along a closed intermediate course and to execute the instruction on the sign as quickly as possible. The sign was positioned such that it was not visible until participants crested a berm. Two high-speed video cameras captured the movements of each participant. RT was defined as the time between when the sign first came into view and when the skier or snowboarder initiated a response (the time of initial ski, snowboard, or body movement away from the original path or arc of the participant). The average RT for skiers and snowboarders was 856 and 1056 ms, respectively. No difference in RT was observed between stopping and turning responses. These data can be used to estimate the limits of performance for an attentive, experienced skier or snowboarder under good environmental conditions.