Abstract: Microfluidic devices have been coupled to mass spectrometers (MS) though their dimensions seem unlikely for marriage. Three types of interface designs are concluded for MS with electrospray ionization while some novel means is also reported to couple microchip to MALDI-TOF. The substrate of microfluidic devices cover both glass and plastic. Various integrated protein preparation and preconcentration procedures together with its trend toward multiplexing are also reviewed.