Abstract: A method and apparatus for generating steam in a forced-through-flow boiler. During operation, water is conveyed through a preheater, an evaporator and, if needed, a superheater. For such a boiler operated with waste heat, the evaporator is divided into at least two sections, a first one of which is supplied with water at the beginning of a start-up phase and is included in a water circulation, while a second one of the evaporator sections, starting from a dry state, is gradually supplied with water in a controlled manner until, in an operating state, water successively flows through both of the evaporator sections. During the start-up phase, and if desired in low load states, a water-steam mixture is conveyed to a separation tank, from which steam is withdrawn and separated feed water is supplied to the preheater.