The paper is intended as a short review of user authentication problem for mobile devices. The emphasis is put on smartphones and tablets, that nowadays are very similar to miniaturized personal computers with much more sensors of various origin. The sensors are described with remarks on their usefulness for user authentication. Deficiencies of traditional user authentication methods based on knowledge are pointed out and the need for new – more secure but also comfortable – user authentication mechanisms is reasoned. Preliminary user authentication systems employing biometric features are discussed and hence the generally unused potential of biometrics for mobile devices is demonstrated.