Micro process engineering is the science of conducting chemical or physical processes (unit operations) inside small volumina, typically inside channels with diameters of less than 1 mm (microchannels) or other structures with sub-millimeter dimensions.
Micro process engineering is therefore not to be confused with microchemistry, which deals with very small overall quantities of matter.
The subfield of micro process engineering that deals with chemical reactions, carried out in microstructured reactors or "microreactors", is also known as microreaction technology.
For example, the length scale of diffusion processes is comparable to that of microchannels or even shorter, and efficient mixing of reactants can be achieved during very short times (typically milliseconds).
Historically, micro process engineering originated around the 1980s, when mechanical micromachining methods developed for the fabrication of uranium isotope separation nozzles were first applied to the manufacturing of compact heat exchangers at the Karlsruhe (Nuclear) Research Center.