The article describes the content of the Paul Karrer Lecture given at the University of Zürich on the 20th of June 2007 by Professor Steven V. Ley. The lecture illustrates the work underway within the Chemistry Department at Cambridge to develop microreactors for flow chemistry applications. These modular, small footprint devices are capable of preparing a wide range of compounds including natural products in up to seven synthesis steps. Products can generally be obtained in high yield and purity without conventional work-up methods using a variety of reaction mixer chips and pre-packed flow tubes of immobilised reagents and scavengers.