The odor sensing system consisting of a chemosensor array and a micro computer has been called as an Electronic-Nose (e-Nose) system. Several types of chemosensors such as a conductometric-type sensor utilizing conductivity change of metal oxide semiconductor by means of chemical reaction of gases, a gravimetric-type sensor utilizing quartz crystal microbalance, resonant frequency change, where the mass change is converted into an optical-type sensor utilizing surface plasmon resonance phenomenon and others have been used for the fabrication of the e-Nose system. In this paper, the principle of chemosensors, key technologies to prepare the intelligent e-Nose system, commercially-available e-Nose system reported so far and applications of e-Nose system are described.