A useful configuration for a MEMS transducer intended to convert acoustic signals into electric signals is a parallel plate capacitor. One plate of the capacitor is rigid, or nearly so, and the other plate is sufficiently flexible to respond to acoustic pressures in the audible range. The two plates are closely spaced with an air gap $g$ between them, and the rigid plate is typically perforated with many holes to avoid pressure buildup in the gap. A fixed bias voltage is maintained between the plates. As the pressure is applied to the flexible film, the gap changes size and a current is induced in order to maintain a fixed bias voltage. The plates of a capacitor are oppositely charged, so they exert an attractive mechanical force on each other. This pressure depends locally on the gap spacing, and the pressure force acts to reduce the gap dimension from its initial value. This is a strongly nonlinear attractive force with the potential for inducing an instability, with the capacitor plates collapsing together. This occurs at a voltage level called the pull-in voltage, a critical system parameter for design. Nonlinearity in the elastic response of the solid can be a significant stabilizing influence. Estimates of the pull-in voltage in terms of material and geometrical parameters of typical systems are discussed.