The equations of motion of the electret foil transducer are derived by applying Lagrange’s equation to the total electrical and mechanical energies of the transducer system. The resulting equations apply for the operation of the transducer both as a sound source and as a sound pickup. An expression for the induced static surface-charge density on the back plate is obtained from these equations modified for the quiescent state. An electrical circuit model of the transducer is given for linear operation.