A method is suggested for determining either the indicial admittance of a linear system, when the response to a known transient is observed, or the shape of the transient, when the admittance is known. A method is also outlined for obtaining the impedance operators, and in particular the impedance coefficients (stiffness, resistance, and inertia). Corrections due to the reaction of the instrument used in measuring the response of the system, or of a restraint to insure stability of the system, can be taken into account when appreciable. Suggested applications are to acoustical and aerodynamical problems.