ESDU PD MEMO 0301 describes procedures for the evaluation and correlation of vapour pressure data. Consideration is restricted to the evaluation of data for the vapour pressures of substances that vaporize without chemical change. After sections on units and the importance of purity of samples used for measurement of vapour pressure, the general shape of the vapour-pressure curve is discussed. Vapour pressure is highly dependent on temperature and evaluation of data invariably involves fitting to an equation relating pressure to temperature; selected equations are listed and recommendations given. The chosen equation may be constrained so that the resultant coefficients yield a curve of the correct shape. The application of these constraints to a Wagner equation is described and a relationship between the equation coefficients given. The effect of weighting is discussed and then follow sections on vapour pressures less than 1000 Pa, the application of the principle of corresponding states, family plots and the vapour pressure of solids. The procedure for evaluation of data is summarised and is illustrated by some examples. The naming of some early equations is discussed. This Memorandum contains much information of value to the specialist with over 150 references to the literature. In addition it gives details of the general principles that govern the preparation of the ESDU vapour pressure Items in the Physical Data Series and serves as a useful introduction to these Items.