Diving medicine, also called undersea medicine, is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how relationships of a diver's fitness to dive affect a diver's safety.

Hyperbaric medicine is a corollary field associated with diving, since recompression in a hyperbaric chamber is used as a treatment for two of the most significant diving related illnesses, decompression illness and arterial gas embolism.

Most diving accidents or illnesses are related to the effect of depth/pressure on gases in the body; examples are decompression sickness, nitrogen narcosis, oxygen toxicity, arterial gas embolism and CO2 retention. Dysbaric osteonecrosis is an example of the effects on the bones and joints of bubbles from decreased pressure in a nitrogen saturated diver.

All divers should be free of conditions and illnesses that would negatively impact their safety and well-being underwater. The diving medical physician should be able to identify, treat and advise divers about illnesses and conditions that would cause them to be at increased risk for a diving accident.

Some reasons why a person should not be allowed to dive are as follows:

Disorders that lead to altered consciousness: conditions that produce reduced awareness or sedation from medication, drugs, marijuana or alcohol; fainting, heart problems and seizure activity.

Disorders that inhibit the "natural evolution of Boyle's Law": conditions or diseases that are associated with air trapping in closed spaces, such as sinuses, middle ear, lungs and gastrointestinal tract. Severe asthma is an example.

Disorders that may lead to erratic and irresponsible behavior: included here would be immaturity, psychiatric disorders, diving while under the influence of medications, drugs and alcohol or any medical disorder that results in cognitive defects.

Besides, other Marine hazards in the ocean that can affect divers include marine life, marine infections, polluted water, ocean currents, waves and surges and man-made hazards such as boats, fishing lines and underwater construction. Diving medical personnel need to be able to recognize and treat accidents from large and small predators and poisonous creatures, appropriately diagnose and treat marine infections and illnesses from pollution as well as diverse maladies such as sea sickness, traveler's diarrhea and malaria.