This chapter discusses the fabrication and characterization of planar amperometric nitric oxide (NO) microsensors and electrochemical NO measurements using the NO microsensors Nitric oxide can be electrochemically oxidized to nitrite and then to nitrate via a three-electron transfer reaction. To detect NO electrochemically, the complete three-electron oxidation of NO to nitrate at the surface of a working electrode is accomplished amperometrically by polarizing the working electrode at a constant potential sufficient for the complete oxidation of NO to nitrate. To fabricate NO sensors, chemically modified electrodes are commonly covered with an additional membrane layer. This increases the selectivity for NO by cutting off other easily oxidized and interfering species. A variety of membranes such as cellulose acetate and nafion have been used to modify the surface of working electrodes via electropolymerization or classic dip coating methods. NO sensors based on chemically modified electrodes have been reported to exhibit both enhanced electrode kinetics and larger currents compared with bare solid electrodes. Effective NO sensors have high selectivity for NO, good sensitivity (low detection limit), linearity, fast response times, and long-term stability. These features can be evaluated using dynamic sensor response curves in which the sensor current (proportional to the concentration of NO) is recorded at a constant working electrode potential (sufficient to oxidize NO to nitrate) while a given amount of NO standard solution is successively added to the stirred deaerated background solution. Distance of the sensor from the NO source plays an important role in quantitative measurements of NO.