1. A computer security service for a computer network accessible by users and comprising services and resources, the computer security service comprising, a policy builder component, comprising a network constituent definition component, for defining user data and services and resources data corresponding to the computer network users, services and resources, and a policy definition component for defining access policies for the computer network users, services and resources, a database component for maintaining user, services and resources data, and access policies, and for providing a set of selected access policies in response to a database query, and a validator component, comprising a request parser for receiving a policy query for service or resource access originated by a network user and for generating a corresponding database query for submission to the database component, and a policy parser for receiving the set of access policies provided by the database component in response to the corresponding database query and for generating a policy decision for communication to the network user based on the set of access policies provided by the database component.