In architecture, a folly is an extravagant, useless, or fanciful building, or a building that appears to be something other than what it is.

The term comes from the fact that such structures have often been dubbed "[name of architect or builder]'s Folly", in the sense of foolishness or madness.

Follies are usually found in parks or large grounds of houses; they may sometimes have been deliberately built to look partially in ruins.

The Parc de la Villette in Paris has a number of modern follies by different architects.

Examples include: