The tragedy of the commons is the concept that, if many people enjoy unfettered access to a finite, valuable resource, such as a pasture, they will tend to overuse it and may end up destroying its value altogether. Even if some users exercised voluntary restraint, the other users would merely replace them, the predictable result being a "tragedy" for all. The concept has been widely discussed, and criticised, in economics, ecology and other sciences.

The metaphorical term is the title of a 1968 essay by ecologist Garrett Hardin. The concept itself did not originate with Hardin but rather extends back to classical antiquity, being discussed by Aristotle. The principal concern of Hardin's essay was overpopulation of the planet. To prevent the inevitable tragedy (he argued) it was necessary to reject the principle (supposedly enshrined in the Universal Declaration of Human Rights) according to which every family has a right to choose the number of its offspring, and to replace it by "mutual coercion, mutually agreed upon".

Some scholars have argued that over-exploitation of the common resource is by no means inevitable, since the individuals concerned may be able to achieve mutual restraint by consensus. Others have contended that the metaphor is inapposite or inaccurate because its exemplar – unfettered access to common land – did not exist historically, the right to exploit common land being controlled by law. The work of Elinor Ostrom, who received the Nobel Prize in Economics, is seen by some economists as having refuted Hardin's claims. Hardin's views on over-population have been criticised as simplistic and racist.

The concept of unrestricted-access resources becoming spent, where personal use does not incur personal expense, was discussed by the philosopher Aristotle, who observed in his Politics that

"That which is common to the greatest number has the least care bestowed upon it. Every one thinks chiefly of his own, hardly at all of the common interest; and only when he is himself concerned as an individual."

In 1833, the English economist William Forster Lloyd published "Two Lectures on the Checks to Population", a pamphlet that included a hypothetical example of over-use of a common resource. This was the situation of cattle herders sharing a common parcel of land on which they were each entitled to let their cows graze.

He postulated that if a herder put more than his allotted number of cattle on the common, overgrazing could result. For each additional animal, a herder could receive additional benefits, while the whole group shared the resulting damage to the commons. If all herders made this individually rational economic decision, the common could be depleted or even destroyed, to the detriment of all.

Lloyd's pamphlet was written after the enclosure movement had eliminated the open field system of common property as the standard model for land exploitation in England (though there remained, and still remain, millions of acres of "common land": see § Commons in historical reality). Carl Dahlman and others have asserted that his description was historically inaccurate, pointing to the fact that the system endured for hundreds of years without producing the disastrous effects claimed by Lloyd.