We have developed a micromachined silicon parallel-plate electroosmotic pump. A silicon nitride dielectric thin film insulates the etched silicon substrate of the micropump, allowing it to operate at electrical potentials up to 500 volts. Standard silicon micromachining processes are used to fabricated the pump. Like all electroosmotic pumps, this micropump has the advantages of simple architecture, no moving parts, low power consumption, and robust operation. A recently-fabricated prototype micropump of this design generates flow rates in excess of 5 μL/min using deionized water as the working fluid. The finite thickness of the electric double layer must be taken into account when modeling the performance of this electroosmotic pump.