Global Positioning System (GPS) is the most recent technology that can be used for many engineering applications. The system, which was originally assumed to assist soldiers, military vehicles, planes and ships, was developed by the US Department of Defence in 1973. For all most last two decades, GPS has been extended to be used for both commercial and scientific societies. GPS has played an important role in earth science investigations for the last decade. Since 1990, GPS has been used for recording continuous deformations of plates in many parts of the world, mainly in USA and Japan. Today there is no any other system that can measure the plate deformations at a precision level of a few milimeters (except for other space based geodetic satellite and interferrometric systems, such as satellite laser ranging (SLR) and very long baseline interferometry(VLBI)) depending of the baseline lengths that can range from a few kilometers to thousands of kilometers. This paper gives an overview of GPS and its applications in earthquake studies in some parts of the world, including the Marmara Region of Turkey.