An emergency ascent is an ascent to the surface by a diver in an emergency. More specifically, it refers to any of several procedures for reaching the surface in the event of an out-of-gas emergency, generally while scuba diving.

Emergency ascents may be broadly categorised as independent ascents, where the diver is alone and manages the ascent by themself, and dependent ascents, where the diver is assisted by another diver, who generally provides breathing gas, but may also provide transportation or other assistance. The extreme case of a dependent ascent is underwater rescue or recovery of an unconscious or unresponsive diver, but this is more usually referred to as diver rescue, and emergency ascent is usually used for cases where the distressed diver is at least partially able to contribute to the management of the ascent.

There are three basic mechanisms for ascending through the water column: Positive buoyancy, hydrodynamic thrust, and force directed through a static external object. Any combination of these can be used.

An emergency ascent usually implies that the diver initiated the ascent voluntarily, and made the choice of the procedure. Ascents that are involuntary or get out of control unintentionally are more accurately classed as accidents. An emergency ascent may be made for any one of several reasons, including failure or imminent failure of the breathing gas supply.

An emergency ascent implies that the dive plan has been abandoned due to circumstances beyond the control of the diver, though they may have been caused by the diver, as is often the case in out-of gas emergencies in scuba diving. Out of gas emergencies are generally the most urgent contingencies in diving, as the available time to deal with the emergency can be measured in minutes or seconds, while most other non-traumatic emergencies allow more time. Other reasons for emergency ascent may include:

Three basic mechanisms are available which can cause a diver to ascend in the water. Any combination of these may be used, and the exact combination will depend on the circumstances of the emergency and to some extent, on the choice of the diver:

The terminology is diverse, and not always used consistently.

Emergency ascents where no assistance from another diver is given.