A policy store associated with a policy decision point of an access control system is updated. The policy decision point is arranged to provide, in response to received decision requests, access control decisions in dependence on one or more policies stored in the policy store, each policy specifying a predetermined access control decision to be provided in response to a particular access request made in respect of a particular attribute or combination of attributes. The policy decision point is associated with at least one policy enforcement point arranged to implement access control in accordance with access control decisions provided by the policy decision point in response to decision requests submitted by the policy enforcement point, the policy enforcement point having associated therewith an attribute store providing data relating to attributes in respect of which access requests have previously been made via the policy enforcement point.