Examples associated with user authentication are described. One example system includes a set of authentication modules. A data store stores data describing disruption ratings of members of the set of authentication modules. A user confidence module maintains a confidence rating that a current user of a device is an authenticated user of the device. The confidence module controls execution of the authentication modules based on the confidence rating and on the disruption ratings of the authentication modules. The user confidence module controls execution of relatively less disruptive authentication modules when the user confidence module is confident that the current user of the device is the authenticated user of the device. The user confidence module maintains the confidence rating based on feedback received from authentication modules.