This is generally directed to identifying unauthorized users of an electronic device. In some embodiments, an unauthorized user of the electronic device can be detected by identifying particular activities that may indicate suspicious behavior. In some embodiments, an unauthorized user can be detected by comparing the identity of the current user to the identity of the owner of the electronic device. When an unauthorized user is detected, various safety measures can be taken. For example, information related to the identity of the unauthorized user, the unauthorized user's operation of the electronic device, or the current location of the electronic device can be gathered. As another example, functions of the electronic device can be restricted. In some embodiments, the owner of the electronic device can be notified of the unauthorized user by sending an alert notification through any suitable medium, such as, for example, a voice mail, e-mail, or text message.