The advent of Global Positioning System (GPS) has revolutionized geodesy with high accuracy, fast speed, simple use, and low cost. This dissertation investigates three topics on volcano and earthquake-related deformation using GPS measurements and models to demonstrate the power of the new generation of geodetic methods. The three topics include the 2002-2003 continued episodic inflation at Long Valley Caldera in eastern California, the coseismic and postseismic response of the energetic 2008 MW 6.4 Achaia-Elia Earthquake in northwest Peloponnese, Greece, and the interseismic megathrust coupling and forearc sliver transport near the Nicoya Peninsula in northwest Costa Rica.