Unlike the Modern Olympic Games, only free men who spoke Greek were allowed to participate in the Ancient Games. They were to some extent "international", though, in the sense that they included athletes from the various Greek city-states. Additionally, participants eventually came from Greek colonies as well, extending the range of the games to far shores of the Mediterranean and of the Black Sea.
In order to be in the games one had to qualify and the athlete had to have one's name written down in the lists. It seems that only young people were allowed to participate, as the Greek writer Plutarch relates that one young man was rejected for seeming too mature, and only after his lover interceded with the king of Sparta, who presumably vouched for his youth, was he permitted to participate. Before being able to participate, every participant had to take an oath in front of the statue of Zeus saying that he had been in training for 10 months.

Athletes running the hoplitodromos
The Olympic games originally contained one event: the stadion (or "stade") race, a short sprint measuring between 180 and 240 metres, or the length of the stadium. The actual length of the race is unknown, since tracks found at archeological sites, as well as literary evidence, provide conflicting answers. Runners had to pass five stakes that divided the lanes: one stake at the start, another at the finish, and three stakes in-between. Since time was not pertinent to winning the stadion, merely passing the finish stake first was enough to earn the victory.