This collection presents the results of many years of research conducted primarily by scientists from the Bulgarian Academy of Sciences on the effects of various extreme factors on the human body. It provides a description of the main hazards encountered in outer space, in the deep sea, and during shipwrecks. It describes the primary reactions of the central nervous system when exposed to these conditions. It also discusses the influence of geomagnetic factors on psychological reactions and emotions. A significant portion of the presented data on physiological characteristics, as well as the methods and tools used to study them under extreme conditions, is being published for the first time. The exploration of new territories, such as space and the world’s oceans, the Earth’s poles, and other planets, depends to a large extent on technological development. However, the ability of humans to reside in these territories and the duration of their stay there are determined by advances in medicine. In future international scientific projects to send people to various points on Earth and in the Universe, medical research will take its rightful place with its unchanging primary task of preserving human health.