The Casimir force between uncharged metallic surfaces originates from quant
um-mechanical zero-point fluctuations of the electromagnetic field. We demo
nstrate that this quantum electrodynamical effect has a profound influence
on the oscillatory behavior of microstructures when surfaces are in close p
roximity (less than or equal to 100 nm). Frequency shifts, hysteretic behav
ior, and bistability caused by the Casimir force are observed in the freque
ncy response of a periodically driven micromachined torsional oscillator.