The book covers propagation of seismic waves on all scales from the global to the local. We start by providing a discussion of the way that the higher frequency body wavesand surface wave components emerge from the normal mode spectra of the Earth for radially stratified structure. Thistreatment provides a formulation that links global and local concepts, which is exploited in later chapters. We introduce the description of seismic wave propagation in terms of reflection and transmission through zones of Earth structure and show how this enables understanding of the physical processes that lead to observed seismic signals.