Techniques are described for authenticating an individual based at least partly on a tremor signature of the individual. Motion data is collected through motion sensor(s), such as accelerometers, gyroscopic motion sensors, and so forth, of a portable computing device that is being held, worn, or is otherwise in contact with the user. Based on the collected motion data, a tremor signature may be determined and compared to a previously determined model for the user, and an authentication determination may be made based on the comparison. The tremor signature may be used in combination with other information to authenticate the user. For example, the tremor signature may be used in combination with location information and/or other biometric data such as a fingerprint scan, image of the user's face, audio recording of the user's voice, the user's heartbeat, a cardiac electrical signature, bio-electrical impedance, and so forth.