An authentication server (AS) is provided which stores authentication details of authorised users, and a list of currently-authenticated users. A number of application servers (APS) are connected to the authentication server (AS), to allow the application servers (APS) to check the current authentication status of a user (T1, T2, T3) which requests service by the application servers (APS). There is also provided a method of authenticating a user for access via a terminal (T1, T2, T3) connected by an Internet Protocol connection to a Web server (APS), said method comprising: storing authentication details of authorised users; performing authentication of a user with reference to said stored authentication details; transmitting an identifier for the user's terminal (T1, T2, T3) to said terminal (T1, T2, T3) for storage thereon, the identifier being transmitted in such a manner that the identifier is transmitted by said user terminal (T1, T2, T3) with document requests directed at said Web server (APS); storing status data indicating said identifier to be an identifier of a terminal (T1, T2, T3) of a currently authenticated user; and allowing said Web server (APS) to access said status data in order to check the authentication status of a user on receipt of a document request containing said identifier.