Heterogeneously catalyzed gas-phase reactions are of great technical and economic importance in the chemical industry. Enormous resources continue to be expended on the development and optimization of catalysts, technical concepts for reactors, and/or operating conditions. Microreaction technology is very well suited to making extremely valuable contributions here in many different ways. On account of the geometric ratios and also the high specific surface areas, heat- and mass-transport limitations can generally be largely excluded or drastically reduced. This allows process conditions that cannot be realized in most conventional apparatus, as is convincingly demonstrated by, for example, the possibility of safely carrying out reactions involving explosive mixtures.