This volume was originally published in 1983. It is concerned with the features of and the laws governing the occurrence of water in the interior of the Earth. Special attention is paid to the origin of the water in the interior of the Earth, the manner in which it moves and its changes of state. The laws which govern the distribution of groundwater are examined from the position of an understanding of the hydrogeological structure and the water-bearing systems. A scheme for the classification of groundwater is given based on the manner in which it is deposited. The deep-lying water and the subsurface water under the seas and oceans are examined together with the traditional subdivisions. The concluding sections are concerned with hydrogeothermics, the principles of hydrogeological zonation and the distribution of groundwater throughout the world. The basic viewpoints are illustrated by copious material from all over the world.