Crowd collapses and crowd crushes are catastrophic incidents that occur when a body of people becomes dangerously overcrowded. When numbers are up to about five people per square meter, the environment may feel cramped but manageable; when numbers reach between eight and ten people per square meter, individuals become pressed against each other and may be swept along against their will by the motion of the crowd. Under these conditions, the crowd may undergo a progressive collapse where the pressure pushes people off their feet, resulting in people being trampled or crushed by the weight of other people falling on top of them. At even higher densities, the pressure on each individual can cause them to be crushed or asphyxiated while still upright.

Such incidents are invariably the product of organizational failures, and most major crowd disasters could have been prevented by simple crowd management strategies. Such incidents can occur at large gatherings such as sporting, commercial, social, and religious events. The critical factor is crowd density rather than crowd size.

Crowd collapses and crushes are often reported incorrectly as human stampedes, which typically occur when a large group of people all try to get away from a perceived risk to life.

One study has calculated that there were 232 deaths and over 65,000 injuries in the ten years between 1992 and 2002 as a result of such incidents, but crowd scientists believe that such casualties are both vastly under-reported and increasing in frequency. One estimate is that only one in ten crowd injuries occurring in doorbuster sales are reported, while many, if not most, injuries at rock concerts go unreported.

The average individual occupies an oval floorspace approximately 30 by 60 centimetres (1 by 2 ft)—0.2 square metres; 2 square feet—and at densities of 1 to 2 per square meter (or per 10 ft²) individuals can move freely without contact. Even if people are moving quickly, at this density one can avoid obstacles, and the chance of a crowd-related incident is minimal. Even at three or four people per square meter, the risk is low; however, at densities of five per square meter, it becomes more difficult for individuals to move, and at higher densities of six to seven per square meter, individuals become pressed against each other and can be unable to move voluntarily. At this point a crowd can begin to behave like a fluid, with individuals moved about by the pressure of those around them, and shockwaves can pass through the crowd as pressures within the crowd change. This can be highly dangerous, although some people actively seek this experience, such as at rock concerts or football matches, where the excitement, camaraderie, and literally "going with the flow" is for some an essential part of the experience, and activities like dancing and moshing are common. The danger inherent in these conditions is that the crowd will collapse in on itself or become so densely packed that individuals are crushed and asphyxiated.

A crowd collapse occurs when a crowd is so dense that each individual is touching others all around and is, to an extent, supported by those around. This can occur whether the crowd is moving or stationary. If a person then falls, the support to those around is lost, while the pressure from those further out remains, causing people to fall into the void. This process is then repeated, causing a bigger void, and will progress until the pressure eases; meanwhile, those who have fallen are at risk of being smothered by the weight of bodies on top or being trampled as the crowd is swept over them. An example of a progressive crowd collapse was the 2015 Mina stampede in Mecca, Saudi Arabia during the Hajj when over 2,400 people were reported to have died.

At even higher densities (approaching ten people per square metre (one per square foot)) a crowd can become so packed that people are crushed together to such an extent they can no longer breathe and are asphyxiated. Such crowd crushes can occur when a moving crowd is funneled into a smaller and smaller space, when it meets an obstacle (such as a dead end, or a locked door), or when an already densely packed crowd has an influx of people, causing a pressure wave toward those at the front of the crowd. In this situation those entering may be unaware of the effect on those in front and continue to press in. Examples of crushes are the Hillsborough disaster in Sheffield, South Yorkshire, England in 1989, the Love Parade disaster in Duisburg, North Rhine-Westphalia, Germany in 2010, the Astroworld Festival crowd crush in Houston, Texas, in 2021, and the Itaewon Halloween crowd crush in Itaewon, Seoul, South Korea in 2022.

Stampede is a loaded word as it apportions blame to the victims for behaving in an irrational, self-destructive, unthinking and uncaring manner, it's pure ignorance, and laziness ... It gives the impression that it was a mindless crowd only caring about themselves, and they were prepared to crush people. In virtually all situations it is usually the authorities to blame for poor planning, poor design, poor control, poor policing and mismanagement.