A sensitive and miniaturized amperometric nitrate (NO3-) sensor for groundwater monitoring was designed, fabricated, and tested. The thin-film microelectrodes were patterned on the silicon substrate with microfabrication techniques. The sensor shows very promising performance, compared to commercial nitrate sensors. A low detection limit of 4 ?M and wide dynamic range of 10 mM with excellent linearity (r2=0.99) are achieved and yet sensor is in a small form factor (4×4×3 cm). Many efforts have been being made to improve the sensor reliability and to realize stand-alone field measurements. Reference electrode was treated with polyurethane coating. An automated sample-handling and sensor calibration system was studied. A miniaturized potentiostat with built-in pumps and valves controller have also been made. In order to overcome short life-time of the thin-film sensing electrode, a palm-sized sensor that consists of Plexiglass housing, macro-scale electrodes, and microfluidic channels have been newly developed. The interference from common ions in groundwater was characterized and major interferents are Sr+, Ca2+, and PO¬43-. An ion specific membrane is employed in the sensor to minimize interference, and selectivity of the sensor is currently being analyzed.