Keystone is a census-designated place (CDP) in Summit County, Colorado, United States. The population was 1,079 at the 2010 census. The Dillon Post Office (ZIP Code 80435) serves Keystone postal addresses. Keystone is the home of the Keystone Resort. Originally constructed in the 1970s by the Ralston Purina Company, it is now owned by Vail Resorts. The Keystone Resortski area occupies 3 separate mountains: Dercum Mountain, North Peak, and The Outback. Recent expansion of terrain and services offers snowcat skiing in Independence Bowl, Bergman Bowl, Erickson Bowl, in addition to the existing North Bowl and South Bowl on Wapiti Peak. The resort has many summer and winter outdoor activities. The winter activities include alpine skiing, snowboarding, tubing, ice-skating, cross-country skiing, horse-drawn sleigh ride dinners, and snowmobiling. The summer activities include hiking, mountain biking, fishing, trail running, golf on two championship courses, and paddle boating on Keystone Lake. The Keystone ski area has: Keystone is only a short drive from other resorts in Summit County - about 10 minutes to Arapaho Basin, 20 minutes to Breckenridge, and 20 minutes to Copper Mountain. Day lift tickets and season passes at Keystone are also honored at Breckenridge and Arapaho Basin.