Whereas the bulk chemical industry has historically sought economic advantage through economies of scale, a paradigm shift has researchers developing systems on smaller scales. Nano-cages and nano-actuators increase selectivity and robustness at the molecular scale. In parallel, micro-contactors with sub-millimetre lateral dimensions are decreasing boundary layers that restrict heat and mass transfer and thus meet the objectives of process intensification with great increases in productivity with a smaller footprint. These contactors continue to serve chemical engineers and chemists to synthesize fine chemicals and characterize catalysts; however, they have now been adopted for sensors in biological and biochemical systems. A bibliometric analysis of articles indexed in the Web of Science in 2016 and 2017 identified five major clusters of research: catalysis and bulk chemicals; nanoparticles; organic synthesis and flow chemistry; systems and micro-fluidics applied to biochemistry; and micro-channel reactors and mass transfer. In the early 1990s, less than 100 articles a year mentioned micro-reactors, while over 943 articles mentioned it in 2017. Here, we introduce micro-reactors and their role in the continuous synthesis of fine chemicals across the various scales to commercialization.