We revisit the concept of marginal stability in glasses and determine its range of applicability in the context of an avalanche-type response to slow external driving. We argue that there is an intimate connection between a pseudogap in the distribution of local fields and crackling in systems with long-range interactions. We classify glassy systems according to the presence or absence of marginal stability, providing a unifying perspective on the phenomenology of systems as diverse as spin and electron glasses, hard spheres, pinned elastic interfaces, and soft amorphous solids undergoing plastic deformation. © 2015 by Annual Reviews.