This article reviews the state of the art of microwave-assisted reactions and the influence of microwaves on mass and heat transfer. The heating behavior of representative test reactions and single substances is compared for heating with microwaves and thermal energy. Similarities and differences between convective heating and dielectric irradiation methods are discussed with regard to the yield, the selectivity, and the enantiomeric purity of the reaction products. Furthermore, prevailing problems related to the scale-up of microwave-assisted reactions are discussed considering the energy absorption of the substances and mixtures to be heated, and their dependence on the energy consumption and the amount of substance.