A method for authenticating a user with respect to a computer system (12, 12a, 12b, 12c), the computer system (12, 12a, 12b, 12c) comprising an authentication system including a user interface and an interface for connecting the computer system (12, 12a, 12b, 12c) to a digital, cellular telecommunication network (22, 22a, 22b), the authentication being carried out by way of a mobile telecommunication terminal, the mobile telecommunication terminal comprising an interface for the communication network (22, 22a, 22b), the computer system (12, 12a, 12b, 12c) comprising a memory in which a communication address for the telecommunication terminal is stored, comprising the following steps: - the user identifies himself or herself with respect to the authentication system of the computer system (12, 12a, 12b, 12c) by entering login information via the user interface; - after successful identification of the user on the authentication system, the authentication system accesses the communication address stored in the memory and transmits an activation request via the communication network (22, 22a, 22b) to the mobile telecommunication terminal; - the mobile telecommunication terminal automatically ascertains at least one piece of information that is required for activating the computer system (12, 12a, 12b, 12c) for the user by way of a program that is stored on the mobile telecommunication terminal and/or by way of an interface of the mobile telecommunication terminal; - the mobile telecommunication terminal automatically checks whether the ascertained information meets at least one predefined criterion that is stored on the mobile telecommunication terminal; said method being characterised by: - the mobile telecommunication terminal transmits activation information for the user on the computer system (12, 12a, 12b, 12c) via the communication network (22, 22a, 22b) to the authentication system if the ascertained information meets the stored criterion; - upon receipt of the activation information, the authentication system issues an authentication signal indicating the authentication of the user to the computer system (12, 12a, 12b, 12c), whereby the computer system (12, 12a, 12b, 12c) is activated for the user. - the user identifies himself or herself with respect to the authentication system of the computer system (12, 12a, 12b, 12c) by entering login information via the user interface; - after successful identification of the user on the authentication system, the authentication system accesses the communication address stored in the memory and transmits an activation request via the communication network (22, 22a, 22b) to the mobile telecommunication terminal; - the mobile telecommunication terminal automatically ascertains at least one piece of information that is required for activating the computer system (12, 12a, 12b, 12c) for the user by way of a program that is stored on the mobile telecommunication terminal and/or by way of an interface of the mobile telecommunication terminal; - the mobile telecommunication terminal automatically checks whether the ascertained information meets at least one predefined criterion that is stored on the mobile telecommunication terminal; - the mobile telecommunication terminal transmits activation information for the user on the computer system (12, 12a, 12b, 12c) via the communication network (22, 22a, 22b) to the authentication system if the ascertained information meets the stored criterion; - upon receipt of the activation information, the authentication system issues an authentication signal indicating the authentication of the user to the computer system (12, 12a, 12b, 12c), whereby the computer system (12, 12a, 12b, 12c) is activated for the user.