Underwater detonations have the potential for serious injury in marine vertebrates such as fishes, reptiles, birds and mammals. The high detonation velocity creates a shock wave. The main reason for injury is the extremely short signal rise time combined with a high overpressure. A negative pressure phase generating cavitation shortly after the peak overpressure can increase organ and tissue damage. Due to surface reflection generating a reversed phase replica of the detonation this phenomenon is very pronounced in shallow waters. Organs most seriously affected by detonations are those with gas/tissue interfaces (e.g., ears, lungs, swim bladders, air sacs, intestines). Observed injuries include disruption of cells and tissues by differential displacement, internal bleeding, embolism and auditory damage. Further, the compression of the thorax by the shock wave initiates a rapid increase in blood pressure which can cause damage in the brain and ears. In order to protect marine life all possible attempts should be made to avoid underwater detonations. For detonations which cannot be avoided due to safety considerations, a number of mitigation measures are presented including bubble curtains, scaring devices, visual and acoustic monitoring, seasonal and spatial planning. However, mitigation measures have varying degrees of efficiency. Low-order detonations are not a real alternative due to the release of toxic munitions constituents to the environment. For each detonation, a proper site- and munitions-specific risk assessment and mitigation strategy must be developed.