The background noise is one of the most important specifications of a microphone because it determines the minimum resolvable sound pressure. Two types of membrane-based microphones will be considered: the condenser and the fiber optic lever (intensity-modulated) microphones. The noise sources in the condenser microphone originate in the preamplifier and the membrane (generated in the acoustic resistance of the air layer). The primary sources in the fiber optic lever microphone are shot noise in the photodetector and electrical Johnson noise in the output amplifier. In both cases the noise sources are modeled, equivalent electrical circuits analyzed, and the results compared to measurements. A brief discussion of 1/f noise, both electrical and mechanical, will be included.