Geothermal energy can be divided into hydrothermal and hot rock resources. The hydrothermal resources consist of steam, hot water and brines in the earth. The hot rock resources are much greater and exceed all the sum of the world's fuel resources, and are practically inexhaustible. Hot rock resources are found at depths from 2 to 10 kilometers (the depths of modern boreholes). The size of various geothermal resources in Lithuania is being determined by measuring well water supply and return temperatures. Geothermal fluids for domestic hot water, heating and electricity generation have been classified respectively as 70/20C, 90/40C and 210/70C (supply/return). Accordingly, potential geothermal resources in West Lithuania are estimated as follows: 1. 70/20C - 146 x 10^9 tonnes of conventional fuel equivalent over an area of 23,600 km2 and 2. 90/40C - 99 x 10^9 tonnes of conventional fuel equivalent over an area of 15,600 km2