The physical principles which provide the basis for fiber optic acoustic sensors (microphones and hydrophones) are discussed for both interferometric and microbend sensor designs. The response of the interferometric microphone is examined from 1 Hz to 50 kHz. Experimental results for a microbend based microphone for low frequency applications are presented. The acoustic response of optical fibers is discussed in relationship to ultra-sonic probes and fiber modulator applications. Finally, a brief description of the status of low frequency fiber optic hydrophones is presented.