Ruins (from Latin ruina 'a collapse') are the remains of a civilization's architecture. The term refers to formerly intact structures that have fallen into a state of partial or total disrepair over time due to a variety of factors, such as lack of maintenance, deliberate destruction by humans, or uncontrollable destruction by natural phenomena. The most common root causes that yield ruins in their wake are natural disasters, armed conflict, and population decline, with many structures becoming progressively derelict over time due to long-term weathering and scavenging.

There are famous ruins all over the world, with notable sites originating from ancient China, the Indus Valley and other regions of ancient India, ancient Iran, ancient Israel and Judea, ancient Iraq, ancient Greece, ancient Egypt, Roman,ancient India sites throughout the Mediterranean Basin, and Incan and Mayan sites in the Americas. Ruins are of great importance to historians, archaeologists and anthropologists, whether they were once individual fortifications, places of worship, ancient universities, houses and utility buildings, or entire villages, towns, and cities. Many ruins have become UNESCO World Heritage Sites in recent years, to identify and preserve them as areas of outstanding value to humanity.