An apparatus for user authentication based on tracked activity includes an activity tracker module, a challenge module, and an authentication module. The activity tracker module is configured to electronically track one or more activities of a user. Electronically tracking the one or more activities includes obtaining information about at least one activity from an electronic device of the user. The challenge module is configured to present an authentication challenge to the user via a user interface for the electronic device. The authentication challenge is based on the one or more electronically tracked activities for the user. The authentication module is configured to determine whether to authenticate the user for access to one or more resources via the electronic device, based on the user's response to the authentication challenge.