Objective-collapse theories, also known as models of spontaneous wave function collapse or dynamical reduction models, are proposed solutions to the measurement problem in quantum mechanics. As with other theories called interpretations of quantum mechanics, they are possible explanations of why and how quantum measurements always give definite outcomes, not a superposition of them as predicted by the Schrödinger equation, and more generally how the classical world emerges from quantum theory. The fundamental idea is that the unitary evolution of the wave function describing the state of a quantum system is approximate. It works well for microscopic systems, but progressively loses its validity when the mass / complexity of the system increases. In collapse theories, the Schrödinger equation is supplemented with additional nonlinear and stochastic terms (spontaneous collapses) which localize the wave function in space. The resulting dynamics is such that for microscopic isolated systems the new terms have a negligible effect; therefore, the usual quantum properties are recovered, apart from very tiny deviations. Such deviations can potentially be detected in dedicated experiments, and efforts are increasing worldwide towards testing them. An inbuilt amplification mechanism makes sure that for macroscopic systems consisting of many particles, the collapse becomes stronger than the quantum dynamics. Then their wave function is always well-localized in space, so well-localized that it behaves, for all practical purposes, like a point moving in space according to Newton's laws. In this sense, collapse models provide a unified description of microscopic and macroscopic systems, avoiding the conceptual problems associated to measurements in quantum theory. The most well-known examples of such theories are: Ghirardi–Rimini–Weber (GRW) model Continuous spontaneous localization (CSL) model Diósi–Penrose (DP) model Collapse theories stand in opposition to many-worlds interpretation theories, in that they hold that a process of wave function collapse curtails the branching of the wave function and removes unobserved behaviour. History of collapse theories The genesis of collapse models dates back to the 1970s. In Italy, the group of L. Fonda, G.C. Ghirardi and A. Rimini was studying how to derive the exponential decay law in decay processes, within quantum theory. In their model, an essential feature was that, during the decay, particles undergo spontaneous collapses in space, an idea that was later carried over to characterize the GRW model. Meanwhile, P. Pearle in the USA was developing nonlinear and stochastic equations, to model the collapse of the wave function in a dynamical way; this formalism