The modern Olympic Games are the world's biggest international sporting events. They feature summer and winter sports events in which thousands of athletes from around the world participate in a variety of athletic competitions. The Olympic Games, open to both amateur and professional athletes, involve more than 200 teams, each team representing a sovereign state or territory. The Games often, but not always, substitute for any world championships during the year in which they take place. The Olympics are staged every four years. Since 1994, they have alternated between the Summer and Winter Olympics every two years during the four-year Olympiad.