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.  
These processes are usually carried out in continuous flow mode, as opposed to batch production, allowing a throughput high enough to make micro process engineering a tool for chemical production. 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.
The unique advantages of microstructured reactors or microreactors are enhanced heat transfer due to the large surface area-to-volume ratio, and enhanced mass transfer. 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). The good heat transfer properties allow a precise temperature control of reactions. For example, highly 
exothermic reactions can be conducted almost isothermally when the microstructured reactor contains a second set of microchannels ("cooling passage"), fluidically separated from the reaction channels ("reaction
passage"), through which a flow of cold fluid with sufficiently high
heat capacity is maintained. It is also possible to change the temperature of microstructured reactors very rapidly to intentionally achieve a non-isothermal behaviour.