Microchannel apparatus comprising a header and plural flow microchannels is described in which orifices connect the header and the flow microchannels. The orifices constrict flow. The ratio of the cross-sectional area of each of the orifices to the cross-sectional area of the flow microchannels connected to the orifices is between 0.0005 and 0.1. Also described is microchannel apparatus for conducting unit operations in which a device comprises two arrays of microchannels, and a unit operation is conducted on a fluid as it passes through the first set of microchannels and into a header where a second unit operation is performed, and then the fluid stream passes into the second array of microchannels where the first unit operation is again performed. Methods of conducting unit operations in the apparatuses are also described.