Seismology has complementary observational and theoretical components, and a thorough understanding of the observations requires a sound theoretical background. This book bridges the gap between introductory textbooks and advanced monographs by providing the necessary mathematical tools and demonstrating how to apply them. Each seismological problem is carefully formulated and its solution is derived in a step-by-step approach. The text includes student exercises (with hints), for which solutions are available on a dedicated website. This website also contains numerous downloadable programs for the computation of reflection and transmission coefficients, for the generation of P and S wave radiation patterns and synthetic seismograms, in infinite media. This book will therefore find a receptive audience among advanced undergraduate and graduate students interested in developing a solid mathematical background to tackle more advanced topics in seismology. It will also form a useful reference volume for researchers wishing to brush up on the fundamentals.