Recent examples of high-pressure organic reactions in (flow) glass microreactors of micrometer dimensions are described. Different types of reactions such as Diels-Alder and nucleophilic aromatic substitution reactions performed in a capillary microreactor gave rate enhancements of 1.5-2.7 at 600 bar. Specifically designed flow glass microreactors, that withstand pressures of 140-690 bar, were used to Study Diels-Alder and oxidation reactions. The use of supercritical fluids gives rise to considerable rate enhancements, while the use of a transparent microreactor allows to visualize phase transitions.