Croatia as a maritime country has a long tradition in diving. Popularity of diving, as a form of sports, professional and recreational activity has recently significantly increased. What makes diving so special compared to most other sports activities, are the conditions in which it is performed. Depth, i.e. high pressure of water, is unnatural environment for men and it requires extreme physical fitness, both for professional and recreational divers. This paper stresses possible consequences of harmful effects diving might have on human body and with more details it deals with effect of some gasses (oxygen, carbon dioxide and nitrogen), which are harmless in the atmospheric conditions at sea level, but however, with the influence of depth their partial pressure increases and causes many harmful effects in a diver’s body like: decompression sickness, neurotoxicity of nitrogen, so called deep sea intoxication, neurotoxicity of oxygen and its free radicals. The paper also deals with hypoxia, condition of reduced partial oxygen in blood, as an extremely dangerous occurrence which may have lethal consequence in only a few minutes. Possible harmful effect of high water pressure on sinuses and middle ear are also described in this paper, as one of pathologies of diving which occur rather often. Because of all the described conditions it is necessary to be well informed about the pathophysiology of diving in order to be able to recognize early signs of harmful effects. That is important because demanding diving conditions often make impossible for medical care to be provided on time or to be provided at all which often results in death. It is because of this reason and because of a more successful prevention that it is necessary to be well informed about medical aspects of this activity before going to dive.