Method for authenticating a user, comprising the steps of a) providing a central server, in communication with at least two authentication service providers (110,120,130) and at least one user service provider; b) associating each authentication service provider with at least one respective available level of authentication; c) receiving, at the central server, a request from the user service provider to authenticate a particular user accessing the user service provider via an electronic device (170,180); d) identifying a minimum level of authentication to use for the said request; e) causing the central server to identify a selected one of said authentication service providers which is associated with at least one allowable level of authentication which is at least as high as the said minimum level of authentication, and which selected authentication service provider is capable of authenticating said particular user at said allowable level of authentication; f) either providing user credential data associated with said particular user directly to the selected authentication service provider, without said user credential data being supplied to the central server, or determining that the selected authentication service provider has an active authentication session for the particular user; and g) causing the selected authentication service provider to authenticate the particular user and to provide an authentication response, wherein step g) further comprises providing access, to the user service provider, to user information previously stored and associated with the particular user with the selected authentication service provider, characterised in that step g) comprises the selected authentication service provider being caused to provide a first access token, using which the said user information can be accessed from the authentication service provider, to the central server, in that step g) further comprises the central server being caused to provide, to the user service provider, a second access token using which the user service provider can request the central server to provide information regarding the user, and in that, upon such request using the second access token, the central server is caused to query, using the first access token, the selected authentication service provider for said user information and provide the user information to the user service provider. a) providing a central server, in communication with at least two authentication service providers (110,120,130) and at least one user service provider; b) associating each authentication service provider with at least one respective available level of authentication; c) receiving, at the central server, a request from the user service provider to authenticate a particular user accessing the user service provider via an electronic device (170,180); d) identifying a minimum level of authentication to use for the said request; e) causing the central server to identify a selected one of said authentication service providers which is associated with at least one allowable level of authentication which is at least as high as the said minimum level of authentication, and which selected authentication service provider is capable of authenticating said particular user at said allowable level of authentication; f) either providing user credential data associated with said particular user directly to the selected authentication service provider, without said user credential data being supplied to the central server, or determining that the selected authentication service provider has an active authentication session for the particular user; and g) causing the selected authentication service provider to authenticate the particular user and to provide an authentication response,