ESDU 74021 is the third in an integrated group of seven that deal with methods of assessing the stability and response of linear systems. It treats those methods of stability analysis that are based on frequency response and presents the Nyquist and Bode presentations for systems involving generalised feedback and the methods of Direct Nyquist, including Hall charts, Nichols charts and the inverse Nyquist diagram for systems involving unity feedback. Appendices describe the use of the Nyquist diagram for a system with pure imaginary roots, the construction of the Bode diagram from standard curves, the calculation of the Nichols chart and the estimation of the roots of a characteristic equation from the Nyquist diagram.