The reserve clause, in North American professional sports, was part of a player contract which stated that the rights to players were retained by the team upon the contract's expiration. Players under these contracts were not free to enter into another contract with another team. Once signed to a contract, players could, at the team's discretion, be reassigned, traded, sold, or released.

In professional sports, as opposed to amateur sports, participants receive payment for their performance. Professionalism in sport has come to the fore through a combination of developments. Mass media and increased leisure have brought larger audiences, so that sports organizations or teams can command large incomes. As a result, more sportspeople can afford to make sport their primary career, devoting the training time necessary to increase skills, physical condition, and experience to modern levels of achievement. This proficiency has also helped boost the popularity of sports. In most sports played professionally there are many more amateur than professional players, though amateurs and professionals do not usually compete.