Many catalogues of astronomical data appear in book form as well as in a machine-readable format. The latter form is popular because of the convenience of handling large bodies of data by machine and because it is an efficient way in which to transmit and make accessible data in books which are now out of print or very difficult to obtain. Some new catalogues are prepared entirely in a machine-readable form and the book form, if it exists at all, is of secondary importance for the preservation of the data.

In this paper comments are given about the importance of prefaces for transmitting the results of a critical evaluation of a body of data and it is noted that it is essential that this type of documentation be transferred with any machine-readable catalogue. The types of error sometimes encountered in handling machine-readable catalogues are noted. The procedures followed in developing the Goddard Cross Index of eleven star catalogues are outlined as one example of how star catalogues can be compared using computers. The classical approach to evaluating data critically is reviewed and the types of question one should ask and answer for particular types of data are listed. Finally, a specific application of these precepts to the problem of line identifications is given.