The scope of extreme-environment diving defined within this work encompasses diving modes outside of the generally accepted no-decompression, open-circuit, compressed-air diving limits on selfcontained underwater breathing apparatus (scuba) in temperate or warmer waters. Extreme-environment diving is scientifically and politically interesting. The scientific diving operational safety and medical framework is the cornerstone from which diving takes place in the scientific community. From this effective baseline, as evidenced by decades of very low DCS incidence rates, the question of whether compressed air is the best breathing medium under pressure was addressed with findings indicating that in certain depth ranges a higher fraction of oxygen (while not exceeding a PC 2 of 1.6 ATA) and a lower fraction of nitrogen result in extended bottom times and a more efficient decompression. Extremeenvironment diving under ice presents a set of physiological. equipment, training and operational challenges beyond regular diving that have also been met through almost 50 years of experience as an underwater research tool. Diving modes such as mixed-gas, surface-supplied diving with helmets may mitigate risk factors that the diver incurs as a result of depth, inert gas narcosis or gas consumption. A close approximation of inert gas loading and decompression status monitoring is a function met by dive computers, a necessity in particular when the diver ventures outside of the single-dive profile into the realm of multi-level, multi-day repetitive diving or decompression diving. The monitoring of decompression status in extreme environments is now done exclusively through the use of dive computers and evaluations of the performance of regulators under ice have determined the characteristics of the next generation of life-support equipment for extreme-environment diving for science. These polar, deep and contaminated water environments require risk assessment that analyzes hazards such as cold stress, hydration, overheating, narcosis, equipment performance and decompression sickness. Scientific diving is a valuable research tool that has become an integral methodology in the pursuit of scientific questions in extreme environments of polar regions, in contaminated waters, and at depth.