This text offers a brief introduction to the dispersion relations as an approach to calculate S-matrix elements, a formalism that allows one to take advantage of the analytical structure of scattering amplitudes following the basic principles of unitarity and causality. First, the case of two-body scattering is considered and then its contribution to other processes through final-state interactions is discussed. For two-body scattering amplitudes, the general expression for a partial-wave amplitude is derived in the approximation where the crossed channel dynamics is neglected. This is taken as the starting point for many interesting nonperturbative applications, both in the light and heavy quark sector. Subsequently crossed channel dynamics is introduced within the equations for calculating the partial-wave amplitudes. Some applications based on methods that treat crossed-channel dynamics perturbatively are discussed too. The last part of this introductory treatment is dedicated to the further impact of scattering amplitudes on a variety of processes through final-state interactions. Several possible approaches are discussed such as the Muskhelishvili-Omnes dispersive integral equations and other closed formulae. These different formalisms are then applied in particular to the study of resonances presenting a number of challenging properties. The book ends with a chapter illustrating the use of dispersion relations in the nuclear medium for the evaluation of the energy density in nuclear matter.

This special volume is dedicated to Geoffrey Chew who passed away on April 12, 2019, at age 94. He is best known as the architect and passionate champion of the bootstrap concept, sometimes called nuclear democracy. His work influenced generations of particle physicists. His passion for physics was an inspiration for his many students and associates. From the Chew-Low theory for meson-nucleon scattering to Analytic S-Matrix, Regge Poles, and Bootstrap principle, his originality left its mark in ways that continue to the present. With contributions from Chew's former collaborators, students, and friends, the book will cover various facets of his life and impact on physics.Contributors include Steven Weinberg, Steven Frautschi, Gabriele Veneziano, Peter Landshoff, Carl Rosenzweig, Basarab Nicolescu, William Frazer, David Gross, John Schwartz, Ling-Lie Chau, Chung-I Tan, Richard Brower, Carleton DeTar, R Shankar, David Kaiser, Fritjof Capra, and others.