Systems and methods for authenticating electronic transactions are provided. The authentication methods employ a combination of security features and communication channels. These security features can be based, for example, on unique knowledge of the person being authenticated, a unique thing that the person has, unique personal features and attributes of the person, the ability of the person to respond, and to do so in a fashion that a machine cannot, and so forth. Methods for enrolling the person prior to authentication are also provided, as well as systems for enrollment and authentication.