In this chapter we introduce the basic notation of seismic waves. In the earth, seismic waves can propagate as longitudinal (P) or as shear (S) waves. For free space, the one-dimensional wave equation is derived. The wave phenomena occurring at a boundary between two layers are discussed, such as Snell’s Law, reflection and transmission. For seismic-exploration purposes, where measurements are taking place at the surface, the different arrivals of direct waves, reflected waves and refracted/head waves are discussed.