Urban exploration (often shortened to UE or urbex) is the exploration of man-made structures, typically abandoned ruins or hidden components of active environments, heavily featuring photography and historical documentation. This adventurous hobby carries significant risks, including physical dangers, the possibility of arrest for trespassing, and potentially dangerous encounters with unauthorized occupants like squatters.

Common exploration sites include abandoned amusement parks, factories, power plants, and hospitals; notable examples are Japan's haikyo (ruins) and Bosnia's Željava Air Base, a vast underground military facility abandoned since 1992. Beyond ruins, urbex also involves "infiltration" of active buildings to access normally unseen areas, or exploring catacombs. An intriguing example is the French group les UX, who clandestinely repaired the Panthéon's clock in 2007. Driven by an appreciation for the beauty of decay and a desire for historical preservation, participants document these forgotten or hidden spaces.