ESDU 70023 gives methods for analysing flight-measured data to determine aircraft climb or descent performance. ESDU 70023 provides a background to the subject as a whole but gives particular emphasis to the development and use of energy methods and the associated measurement of specific excess power which has applications to acceleration and manoeuvring as well as climb performance. The use of these methods does not, however, preclude the interpretation of results in simple, rate-of-climb terms. Also described are test techniques used for determining the flight conditions for point-performance optima with particular attention given to the use of level accelerations. Corrections to measured data and "reduction" formulae and non-dimensional methods for predicting performance in different conditions are presented elsewhere in this Series. The synthesis of practical climb techniques and the scheduling of performance for operational use are also discussed. A limited amount of guidance on sources of data for, and typical values of, wind and wind gradients is given.