The article presents a view on the present knowledge of the influence of microwaves on reactions, material-, and heat transfer. The heating behavior of representative test reactions and material systems in a microwave field is compared to that under thermal energy, Common features and differences between the convective and dielectrical irradiation methods are discussed using the yield, selectivity, and optical purity of the products as main criteria. The energy absorption of substances and substance mixtures in dependence of the input energy and substance quantity is discussed in view of recent problems with the scaling-up of microwave-supported reactions.