The Olympic Games celebrate an Olympiad, or period of four years. The first Olympiad of modern times was celebrated in 1896, and subsequent Olympiads are numbered consecutively, even when no Games take place (as was the case in 1916, 1940, and 1944).

Olympic Winter Games have been held separately from the Games of the Olympiad (Summer Games) since 1924 and were initially held in the same year. In 1986 the IOC voted to alternate the Winter and Summer Games every two years, beginning in 1994. The Winter Games were held in 1992 and again in 1994 and thereafter every four years; the Summer Games maintained their original four-year cycle.

The maximum number of entries permitted for individual events is three per country. The number is fixed (but can be varied) by the IOC in consultation with the international federation concerned. In most team events only one team per country is allowed. In general, an NOC may enter only a citizen of the country concerned. There is no age limit for competitors unless one has been established by a sport’s international federation. No discrimination is allowed on grounds of “race,” religion, or political affiliation. The Games are contests between individuals and not between countries.

The Summer Olympic program includes the following sports: aquatics (including swimming, synchronized swimming, diving, and water polo), archery, athletics (track and field), badminton, basketball, boxing, canoeing and kayaking, cycling, equestrian sports, fencing, field hockey, football (soccer), golf, gymnastics (including artistic, rhythmic, and trampoline), team handball, judo, modern pentathlon, rowing, rugby, sailing (formerly yachting), shooting, table tennis, tae kwon do, tennis, triathlon, volleyball (indoor and beach), weightlifting, and wrestling. Women participate in all these sports, whereas men do not compete in synchronized swimming or rhythmic gymnastics. The Winter Olympic program includes sports played on snow or ice: biathlon, bobsledding, curling, ice hockey, ice skating (figure skating and speed skating), luge, skeleton sledding, skiing, ski jumping, and snowboarding. Athletes of either gender may compete in all these sports. An Olympic program must include national exhibitions and demonstrations of fine arts (architecture, literature, music, painting, sculpture, photography, and sports philately).

The particular events included in the different sports are a matter for agreement between the IOC and the international federations. In 2005 the IOC reviewed the summer sports program, and members voted to drop baseball and softball from the 2012 Games. While sports such as rugby and karate were considered, none won the 75 percent favourable vote needed for inclusion. In 2009 the IOC voted to add women’s boxing to the 2012 program, as well as golf and rugby sevens to the 2016 program. Baseball and softball were added back to the program for the 2020 Games, and karate, skateboarding, sport climbing, and surfing were also added.

To be allowed to compete, an athlete must meet the eligibility requirements as defined by the international body of the particular sport and also by the rules of the IOC.

In the final decades of the 20th century there was a shift in policy away from the IOC’s traditionally strict definition of amateur status. In 1971 the IOC decided to eliminate the term amateur from the Olympic Charter. Subsequently the eligibility rules were amended to permit “broken-time” payments to compensate athletes for time spent away from work during training and competition. The IOC also legitimized the sponsorship of athletes by NOCs, sports organizations, and private businesses. In 1984 some of the world’s best athletes were still banned from the Games because they competed for money, but in 1986 the IOC adopted rules that permit the international federation governing each Olympic sport to decide whether to permit professional athletes in Olympic competition. Professionals in ice hockey, tennis, soccer, and equestrian sports were permitted to compete in the 1988 Olympics, although their eligibility was subject to some restrictions. By the 21st century the presence of professional athletes at the Olympic Games was common.

At the 1960 Rome Olympics, a Danish cyclist collapsed and died after his coach had given him amphetamines. Formal drug tests seemed necessary and were instituted at the 1968 Winter Games in Grenoble, France. There only one athlete was disqualified for taking a banned substance—beer. But in the 1970s and ′80s athletes tested positive for a variety of performance-enhancing drugs, and since the ′70s doping has remained the most difficult challenge facing the Olympic movement. As the fame and potential monetary gains for Olympic champions grew in the latter half of the 20th century, so too did the use of performance-enhancing drugs. Tests for anabolic steroids and other substances improved, but so did doping practices, with the design of new substances often a year or two ahead of the new tests. When 100-metre-sprint champion Ben Johnson of Canada tested positive for the drug stanozolol at the 1988 Summer Games in Seoul, South Korea, the world was shocked, and the Games themselves were tainted. To more effectively police doping practices, the IOC formed the World Anti-Doping Agency in 1999. There is now a long list of banned substances and a thorough testing process. Blood and urine samples are collected from athletes before and after competition and sent to a lab for testing. Positive tests for banned substances lead to disqualification, and athletes may be banned from competition for periods ranging from a year to life. Yet, despite the harsh penalties and threat of public humiliation, athletes continue to test positive for banned substances; in 2016, for instance, the entire Russian track-and-field team was banned from that year’s Rio de Janeiro Olympics after a wide-ranging state-supported doping ring was uncovered.