Unexploded ordnance (UXO, sometimes abbreviated as UO) and unexploded bombs (UXBs) are explosive weapons (bombs, shells, grenades, land mines, naval mines, cluster munition, and other munitions) that did not explode when they were deployed and remain at risk for detonation, sometimes many decades after they were used or discarded. When unwanted munitions are found, they are sometimes destroyed in controlled explosions, but accidental detonation of even very old explosives might also occur, sometimes with fatal consequences.

For example, UXO from World War I continues to be a hazard, with poisonous gas filled munitions still a problem. UXO does not always originate from conflict; areas such as military training bases can also hold significant numbers, even after the area has been abandoned.

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 being the most affected age group. An average estimate of 50% of deaths occur within hours of the blast. In recent years, mines have been used increasingly as weapons of terror; especially against local populations, such as in the Syrian civil war.

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) may enter soil and groundwater, thereby contaminating the water supply, likewise with preventing agrarian uses such as farming and food distribution.

Unexploded ordnance, no matter how old, may explode. It might seem that the dangers of UXO decrease over time, but this is not always the case. Corrosion and damages sustained on impact pose significant difficulties to defuse UXO safely and also make the consequences of defusion harder to predict. Mixed explosive agents might separate or migrate over time and leave contact explosives like nitroglycerine at random places in the shell. Sometimes components of the original explosives, in the presence of moisture, can form new explosive compounds with the metals in the shells like picrates that can leave a shell highly explosive, even when it is defused and the detonator destroyed or removed.^{[citation needed]}

Even if unexploded ordnance does not explode, environmental pollutants are released as it degrades. The toxic compounds and heavy metals can contaminate water and soil over time. Recovery, particularly of deeply-buried projectiles, is difficult and hazardous—jarring may detonate the charge. Once uncovered, explosives can often be transported safely to a site where they can be destroyed; if this is not possible, they must be detonated on site which might require evacuation of the surrounding area.^{[citation needed]}

Unexploded ordnance dating as far back as the mid-19th century still poses a hazard worldwide, both in current and former combat areas and at military firing ranges. A major problem with unexploded ordnance is that over the years, the detonator and main charge deteriorate to such an extent that they frequently become more sensitive to disturbance and therefore more dangerous to handle. Construction works may disturb unsuspected unexploded bombs, which may then explode. Forest fires may be aggravated if buried ordnance explodes. Heat waves, causing the water level to drop severely, may increase the danger of immersed ordnance. There are countless examples of people tampering with unexploded ordnance that is many years old, often with fatal results. For this reason, it is universally recommended that unexploded ordnance should not be touched or handled by unqualified persons. Instead, the location should be reported to the local police so that bomb disposal or Explosive Ordnance Disposal (EOD) professionals can render it safe.

Although professional EOD personnel have expert knowledge, skills and equipment, they are not immune to misfortune because of the inherent dangers: in June 2010, construction workers in Göttingen, Germany discovered an Allied 500-kilogram (1,100 lb) bomb dating from World War II buried approximately 7 metres (23 ft) below the ground. German EOD experts were notified and attended the scene. Whilst residents living nearby were being evacuated and the EOD personnel were preparing to disarm the bomb, it detonated, killing three of them and severely injuring six others. The dead and injured each had over 20 years of hands-on experience, and had previously rendered safe between 600 and 700 unexploded bombs. The bomb which killed and injured the EOD personnel was of a particularly dangerous type because it was fitted with a delayed-action chemical fuze (with an integral anti-handling device) which had not operated as designed, but had become highly unstable after over 65 years underground. The type of delayed-action fuze in the Göttingen bomb was commonly used: a glass vial containing acetone was smashed after the bomb was released; the acetone was intended, as it dripped downwards, to disintegrate celluloid discs holding back a spring-loaded trigger that would strike a detonator when the discs degraded sufficiently after some minutes or hours. These bombs, when striking soft earth at an angle, often end their trajectory not pointing downwards, so that the acetone did not drip onto and weaken the celluloid; but over many years the discs degraded until the trigger was released and the bomb detonated spontaneously, or when weakened by being jarred.