Reaction-diffusion (RD) processes are common throughout nature, which uses them routinely to build and control structures on length scales from molecular to macroscopic. At the same time, despite a long history of scientific research and a significant level of understanding of the basic aspects of RD, reaction-diffusion has remained an unrealized technological opportunity. This review suggests that RD systems can provide a versatile basis for applications in micro- and nanotechnology. Straightforward experimental methods are described that allow precise control of RD processes in complex microgeometries and enable fabrication of small-scale structures, devices, and functional systems. Uses of RD in sensory applications are also discussed.