Magnetospheric substorms encompass a myriad of dynamical phenomena resulting from the coupling of the solar wind with the Earth’s magnetic field and the ionosphere. Although external solar wind conditions conducive to substorm development are generally regarded as identified, the precise cause for substorm onset remains an outstanding unresolved problem. The contending theories include externally driven magnetic reconnection, magnetosphere–ionosphere coupling, thermal catastrophe, macro- and microinstabilities (such as interchange/ballooning, Kelvin–Helmholtz, tearing, lower-hybrid drift, modified two-stream, and ion-Weibel instabilities). The conventional approach to substorm modeling is to describe the entire substorm episode on the basis of a single mechanism. After a brief review and critique of existing substorm models, a new paradigm is introduced in which compatible features among different models are synthesized into a single one which is superior to any individual model in consistency with observations. This new paradigm advocates that substorms may be triggered by more than one mechanism and also that the full set of substorm phenomena may only be accounted for by more than one physical process. Some unresolved substorm problems amenable to immediate future investigation are also identified.