Publisher Summary This chapter provides an introduction to paleoseismology. Paleoseismology is the study of prehistoric earthquakes especially their location, timing, and size. Paleoseismology differs from the general studies of slow to rapid crystal movements during the late Cenozoic in its focus on the almost instantaneous deformation of landforms and sediments during earthquakes. Paleoseismology supplements historic and instrumental records of seismicity by characterizing and dating large prehistoric earthquakes. For the most part, the paleoseismic record is a record of large (magnitude, M > 6.5) or great (M > 7.8) earthquakes, because geologic evidence of small and moderate-sized earthquakes is rarely created or preserved near the surface. Evidence of past earthquakes can range from local deformation of the ground surface along a crystal fault, to indicators of the sudden uplift or subsidence of large regions above a plate-boundary fault, and to stratigraphic or geomorphic effects of strong ground shaking or tsunamis far from the seismogenic fault. A characteristic of most such features is that they formed instantaneously during or immediately after an earthquake. Paleoseismology is a subdiscipline within the much broader fields of neotectonics and active tectonics. Neotectonics, or the study of crystal movements during the late Cenozoic, encompass a wide spectrum of topics ranging from satellite and laser-ranging measurements of past year's movements, to isostatic rebound after glacier retreat, and to fission-track dating of rates of mountain uplift over millions of years. Active tectonics includes many of these types of crystal movements, but they are limited in its time-frame to studies of tectonic movements that are expected to occur within a future time span of concern to society.