A wearable computing device is authenticated using bone conduction. When a user wears the device, a bone conduction speaker and a bone conduction microphone on the device contact the user's head at positions proximate the user's skull. A calibration process is performed by transmitting a signal from the speaker through the skull and receiving a calibration signal at the microphone. An authentication process is subsequently performed by transmitting another signal from the speaker through the skull and an authentication signal is received at the microphone. In the event that frequency response characteristics of the authentication signal match the frequency response characteristics of the calibration signal, the user is authenticated and the device is enabled for user interaction without requiring the user to input any additional data.