Content items or data defining access to content items for provision to one or more users is respectively maintained. Respective content access data for those users from which content items or data defining access to content items are received is changed and a sub-set of the content items or data defining access to content items is provided to a user, the members of the sub-set being determined in dependence on the content access data of the user. Each content item or set of data defining access to a content item has a property having a value, and the content access data includes a content access value relating to the property, wherein the sub-set of the content items or data defining access to content items is determined in dependence on the respective values.