Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for proximity-based access. In some implementations, a computing device detects an attempt to access the computing device while the computing device is in a secured state. In response to detecting the attempt to access the computing device, the computing device sends a first message to a server system over a network. After sending the message, the computing device receives a second message from the server system over the network, the second message comprising authentication data for the computing device. The computing device determines that a mobile device that was previously designated as an authentication factor for accessing the computing device is located within a predetermined level of proximity of the computing device, and the computing device grants access to the computing device.