Design of a separation unit requires real information about the phase equilibrium of the system being handled. Accurate equilibrium data allows the best design from a thermodynamic viewpoint and contributes to a better knowledge about the behaviour of fluids and their mixtures. The principles behind the concept of thermodynamic consistency are presented and discussed. The present state of the art shows that no definite test is available for insuring the quality of the measured values. The main available procedures for testing the consistency of vapour-liquid equilibrium (VLE) data at constant temperature or pressure are reviewed and analysed and recommendations provided for their proper use, for the presentation of VLE results, and also some possible means for determining their quality. Suitable examples are provided about the adequate use of the available tests and about their misuse. (C) 2016 Elsevier Ltd.