INTRODUCTION This Item is the first in a group dealing with methods of assessing the stability and response of linear systems. It introduces the mathematical description of linear systems and the formulation of the characteristic equation. The assessment of stability from a knowledge of the roots of the characteristic equation@ or from its coefficients by means of the Routh-Hurwitz Criteria@ is discussed and the calculation of the modes of motion is explained. The concept of a system transfer function is introduced together with its application to systems with feedback. 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. Part III@ Item No. 74021@ treats the methods of system assessment that are based on frequency response. 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.