A general account is given of the principles used in measuring contact electrification, and of the difficulties due to discharges in air and to unknown contact areas. The nature is discussed of the information which we might hope to obtain from work on this subject, and a list is made of the various mechanisms which have been proposed in the past, together with one new hypothesis. The suggestion is made that these are by no means mutually exclusive, and a simple form of theory is worked out to illustrate how several of them might be expected to operate together.