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 NambüGoldstone bosons, nonlinear realization of internal and spacetime symmetries..