Multiphase microreactor applications are demonstrated with a broad range of cases studies, including carbonylation and fluorination as well as synthesis of solid colloidal nanoparticles and quantum dots. These microchemical systems enable rapid, continuous discovery and development of new products with less environmental impact. Moreover, by integrating sensors for flow, temperature, and chemical composition it is possible to use the sensor information in on-line optimization and control for identification of optimum operating conditions. Microfabrication creates microreactors that provide a hundredfold or more improvement in mass transfer and can be run safely at the benchtop at elevated pressures. Advances in methods for monitoring and control of multiphase flow regimes make it feasible to scale microreactors while treating the resource and safety advantages of microreaction technology. Microreactor synthesis will perhaps have the greatest impact for conditions requiring challenging operating conditions (high pressures and temperatures) and for on-demand, on-site production. The choice of structural materials must have properties that do not compromise design or ability to operate under those conditions. With excellent physical properties of silicon related materials and the readily available fabrication infrastructure, silicon-based systems offer opportunities and flexibility in design of integrated chemical microsystems. Additional attractive characteristics are the wide pressure (0 – 300 atm) and temperature ranges (-80 - 1000º C) feasible with silicon devices, their compatibility with strong acids, bases, and solvents, the integration of optical and electrical sensors, and the ready incorporation of heaters and coolers. We demonstrate applications that take advantage of the elevated pressure and temperature capabilities of silicon based microreactors.