Ruins are the remains of a piece of man-made architecture. The term is used to describe a structure that was at one time whole but has, with the intervention of weathering and lack of maintenance, become wrecked by time. There are basic types of ruins that can be found in the world. Historical ruins, like those found at Rome and Athens, have been unearthed through the work of archaeologists. Modern ruins, such as abandoned buildings in large metropolitan areas, are discovered by urban exploration. The latter type of study is far less legalized than the latter.

In Christian iconography, the Nativity of Christ has often been depicted in a setting of grand ruins, symbolising the new Temple (the body of Christ) that rises on the site of the old.

Popular adventure films, such as the Indiana Jones trilogy or the Mummy films, rely on ancient relics and artifacts found in ruins. The idea of the ruin has become prominent in popular culture as a device with which to add suspense and in many cases horror.

Architectural follies were sometimes built as intentional ruins.

In the Third Reich, Adolf Hitler and Albert Speer designed buildings calculated to have a high "ruin value " for posterity.