The theory and models of type-II supernovae are discussed including results from non-spherical core collapse calculations. The review mainly focuses on the basic physical mechanism of spherically symmetric type-II supernova explosions including a discussion of the influence of the nuclear equation of state on the explosion mechanism. The status of the field is that there is general agreement that, when the detailed effects of neutrino transport are taken into account, available initial models cannot explode by the prompt mechanism, no matter what the correct equation of state is.