The material in this volume provides the basic theory necessary to understand the principles behind imaging the subsurface of the Earth using reflection and refraction seismology. For reflection seismology, the end product is a "record section" from a collection of "wiggly traces" that are recorded in the field from which information about the properties of subsurface structure and rock can be derived. For the most part, the principles of imaging are the same regardless of the depth to the target; the same mathematical background is necessary for targeting a shallow water table as for investigating the base of the earth''s continental "crust" at a depth of 30-50 km.