Certificate policies play a central role in public key infrastructures, since they are the basis for the evaluation of trust in binding between a key and a subject in a public key certificate. The absence of common ways of formally specifying details of policies is a source of difficulty in the operation of global public key infrastructures. In this paper, the problem of the formalisation of certificate policies is discussed and a format for their formal presentation is proposed. Results from the formatting and comparison of existing certificate policies from several well-known certification authorities are also presented. 1999 Elsevier Science B.V. All rights reserved.