Unexploded ordnance (UXO, sometimes abbreviated as UO), unexploded bombs (UXBs), and explosive remnants of war (ERW or ERoW) are explosive weapons (bombs, shells, grenades, land mines, naval mines, cluster munition, and other munitions) that did not explode when they were employed and still pose a risk of detonation, sometimes many decades after they were used or discarded. UXO does not always originate from wars; areas such as military training grounds can also hold significant numbers, even after the area has been abandoned. UXO from World War I continue to be a hazard, with poisonous gas filled munitions still a problem. When unwanted munitions are found, they are sometimes destroyed in controlled explosions, but accidental detonation of even very old explosives also occurs, sometimes with fatal results.

Seventy-eight countries are contaminated by land mines, which kill or maim 15,000–20,000 people every year. Approximately 80% of casualties are civilian, with children the most affected age group. An estimated average of 50% of deaths occurs within hours of the blast. In recent years, mines have been used increasingly as weapons of terror against local civilian populations, specifically.

In addition to the obvious danger of explosion, buried UXO can cause environmental contamination. In some heavily used military training areas, munitions-related chemicals such as explosives and perchlorate (a component of pyrotechnics and rocket fuel) can enter soil and groundwater.