ESDU 86021 is a comprehensive introduction to the concept and use of derivatives in determining aircraft stability. After explaining a systematic method for expressing aerodynamic forces, moments and derivatives in dimensionless form, the equations of motion for an aircraft treated as a rigid body are developed and the linearised small-perturbation form introduced. The conditions are outlined for the separation of the equations into a lateral and a longitudinal set, and a detailed study is made of the behaviour of an aircraft disturbed with controls fixed from straight symmetric level flight. A comprehensive practical worked example illustrates the use of the techniques. A comparison is made between the ISO notation for derivatives used by ESDU and the most widely used notation in the US, equations are given for deriving the longitudinal derivatives from the force and moment coefficients, and reference is included to all the ESDU validated data for lateral derivatives. Also included is the traditional treatment of static stability and manoeuvre margins, and the relation between static and dynamic stability is discussed. Finally, a description is given in simple terms of commonly occurring lateral modes of an aircraft following a disturbance.