INTRODUCTION It is a good practice to define "Deep diving techniques" relatively to conventional Diving. Conventional diving is diving to 150 m in the UK sector of the North Sea There, everything is well defined and organized regulations, qualifications, medical procedures, diving instructions and equipment At these depths, there is no doubt that divers are efficient, their time is unlimited, and their services are affordable. They cumulate approximately 30,000 hours of work at bottom per year for a commercial diving company like Comex. However, in reviewing the offshore industry on a world wide basis, it is becoming increasingly clear that operational diving is gradually moving to deeper waters Todays, deep commercial diving is carried out in excess of 200 m in the Norwegian Trench, in the Campos basin in Brazil and in the Gulf of Mexico There, everything becomes more difficult and complicated divers must be specially selected and trained, physiology limits their interventions high performance equipment is required and specification are lacking. Deep divers are still efficient, but their time is limited and their services are expensive They cumulate approximately 300 hours of work at bottom per year for Comex which runs routine deep operations in Brazil. However, even if this only represents a fraction of the diving activity, it is the intention of this paper to show, using Comex experience in Brazil, that deep diving is much innovative, instructive and is very important for the future of diving operations. SCIENTIFIC BASIS OF DEEP DIVING As usual, research was initiated much in advance of any operational needs and the first deep diving experience came from onshore simulated experimental dives. During the pioneering work of the 60 s it was discovered that deep divers are exposed to 3 types of environmental stresses. the High Pressure Nervous Syndrome (HPNS) which is related to the effects of the pressure on the central nervous system. It appears at around 200 m and increases with depth, causing psychomotor disturbances that may impair divers' efficiency the density of the breathing gas, which also increases with depth, and induces ventilation efforts that reduce diver's work capacity, long confinement period in saturation chambers (thermal stresses, bad sleep, lack of appetite) that induces fatigue and body weight losses Special diving techniques had to be developed before deep diving operations became Possible COMEX EXPERIMENTAL PROGRAMMES In the 70's, research dives were carried out in several countries, originally largely by military founding, but latterly by scientific and commercial organizations Comex had a prominent role in this research work with its PHYSALIE, SAGITTAIRE, JANUS and HYDRA experimental programmes These various programmes tried to use the properties of the different diluents of the diver's breathing gas to overcome the environmental stresses. Three diving techniques were successively introduced. In the early 70', deep dives were conducted with heliox only Helium has a low molecular weight but no anti-HPNS property and the pressurization must be slow In 1972, Comex divers reached 610 m during the PHYSALIE VI dive on heliox after 7 5 days of pressurization.