This paper examines the occurrence of outbursts, coal bursts and rock bursts in underground mining. It then examines them from the viewpoint of the failure mechanism and velocity of particles. The presence of gas is extremely important as it can both act as the fluid pressure component of effective stress leading to failure, and as a substantial energy source in projecting material during a burst. For gas to drive a dynamic event it must be present as free gas held in voids within the rock or coal mass. This can pre-exist the burst event or be released from desorbing gas during it. The latter is typically part of a multi-stage outburst event which can reach very large proportions. Rock and coal bursts are driven by strain energy release and can vary in form. These tend to be single events associated with failure and strain energy release. It is also possible to conceptually have hybrid events where the failure of the mass and the energy release are due to both stress and expanding gas.