A method, implemented by a computer-implemented authentication system, for authenticating a user attempting to access a target component of a computer system, the method comprising: a) Receiving, from a first user system via a computer network, user authentication information and a network address identifying the first user system within the computer network; b) obtaining (S407, S607) at least one data item of contextual information indicative of a property of an environment of a wireless communications device associated with the user authentication information, the wireless communication device and the first user system being different devices; c) authenticating (S403, S409) the user based on at least the user authentication information; and d) subject to successful authentication, granting access to the target component and maintaining a data record indicative of a security context comprising the received network address and the received contextual information; wherein the method further comprises: - determining, after repeated successful access attempts from one or more recurring network addresses, and with a given recurring set of contextual information, and based on one or more selection criteria, at least one recurrent security context from a plurality of stored data records, each data record being indicative of a stored security context comprising one or more network addresses of a user system and received contextual information associated with said one or more network addresses, and each stored security context having associated with it data about one or more previous occurrences of said stored security context; wherein the recurrent security context comprises one or more recurring network addresses and the recurring contextual information which have repeatedly been associated with successful access attempts; and - using (S410, S412) the determined recurrent security context in a subsequent decision whether to grant a subsequent request by said user to access the target component. a) Receiving, from a first user system via a computer network, user authentication information and a network address identifying the first user system within the computer network; b) obtaining (S407, S607) at least one data item of contextual information indicative of a property of an environment of a wireless communications device associated with the user authentication information, the wireless communication device and the first user system being different devices; c) authenticating (S403, S409) the user based on at least the user authentication information; and d) subject to successful authentication, granting access to the target component and maintaining a data record indicative of a security context comprising the received network address and the received contextual information; - determining, after repeated successful access attempts from one or more recurring network addresses, and with a given recurring set of contextual information, and based on one or more selection criteria, at least one recurrent security context from a plurality of stored data records, each data record being indicative of a stored security context comprising one or more network addresses of a user system and received contextual information associated with said one or more network addresses, and each stored security context having associated with it data about one or more previous occurrences of said stored security context; wherein the recurrent security context comprises one or more recurring network addresses and the recurring contextual information which have repeatedly been associated with successful access attempts; and - using (S410, S412) the determined recurrent security context in a subsequent decision whether to grant a subsequent request by said user to access the target component.