Recent research results on microchannel reactors are reviewed with particular reference to their applications and use as a cost effective tool during process development tasks. The high surface to volume ratio, efficient heat and mass transfer characteristics, vastly improved fluid mixing etc., allow precision control of reaction with improved conversions, selectivities and yields of desired products. The reaction times are shorter as compared to the conventional reactors with less degradation and side products. Scalability and optimization are also significantly easier. The article reviews variety of applications to catalysis, polymers, advanced materials, biosystems, organic reaction etc., with emphasis on the related engineering science issues. Microchannel, microfluidic, reactor chip, microlithography, nanotecnology.