The limitations to the accuracy of an inertial navigator are first of all the inherent accuracy of the inertial components—gyros and accelerometers; secondly the accuracy with which these are controlled, i.e. by power supplies, and electronic circuitry; thirdly the accuracy of the associated navigation computer. All these three can be checked during manufacture and by laboratory testing of components and systems. However, there remains the effect of the aircraft environment on accuracy. It is possible to simulate some of the environment, for example vibration and temperature, in the laboratory, but it is not possible to simulate realistic accelerations, velocities and displacements, and so flight trials form an essential part of the evaluation of an inertial system. Flight trials may be preceded by trials in a land vehicle on grounds of economy, but such trials are of limited value because of the limited velocities and displacements available. In this paper we shall attempt to summarize our experience at R.A.E. and A. & A.E.E. of laboratory, van and flight testing of inertial navigators which has been acquired in joint trials over the past five or six years. During the next twelve months we shall be associated in trials of three further inertial navigators, the Elliott E5, the Sperry SGN 10 and the Ferranti FMP-B, and we shall also describe in this paper some of the techniques which will be used for the first time in these trials. This is not intended to be an exhaustive treatment of system testing, but will be mainly confined to the systems and tests of which we have practical experience.