INTRODUCTION This Item is the third in a group dealing with methods of assessing the stability and response of linear systems. Part I@ Item No. 74019@ introduces the mathematical description of linear systems@ both with and without feedback@ and introduces the concept of stability. Part II@ Item No. 74020@ considers methods of graphical presentation that provide the means of displaying the effects on the characteristics of the system of varying some of its parameters. The present Item treats the methods of stability analysis that are based on frequency response@ including the Nyquist@ Bode and Nichols presentations. Part IV@ Item No. 80002@ considers the specification and measures of performance that may be used in conjunction with these methods of assessment. Part V@ Item No. 80018@ deals with the function of the different system elements and with design techniques used to construct a system with the required characteristics@ and Part VI@ Item No. 81008@ considers the factors that@ in particular circumstances@ affect the selection of a specific method of system assessment. The frequency response of a system is its steady-state response to a constant-amplitude sinusoidal input and it is shown in Section 5 that the mathematical function used in the techniques of this Item represents that response for a stable system. For that reason@ the techniques are often said@ as here@ to be based on frequency response. However@ they can also be applied to unstable systems when the mathematical function cannot represent the frequency response and the term has therefore been avoided in the following Sections.