This open access book is about spontaneous symmetry breaking, which is a classic area of theoretical physics that lies at the core of many fascinating phenomena such as ferromagnetism, superfluidity, superconductivity, or the Higgs mechanism. The book brings an up-to-date overview of spontaneous symmetry breaking and of modern effective field theory description thereof. The topics covered include the classification of Nambu-Goldstone bosons, nonlinear realization of internal and spacetime symmetries and the construction of the corresponding effective actions, and selected applications. With in-depth exposition of conceptual foundations and numerous illustrative examples, the book is accessible to anybody having taken a basic course on quantum field theory. It serves as a self-contained text for graduate students and junior researchers in diverse areas of physics, but also as a useful reference for experts.