Nowadays, users are starting to develop a symbiotic bound with their smartphones. Users continuously carry these devices and use them for daily communication activities and social network interactions. On the other hand, a smartphone is equipped with sensors that can infer not only information about the context (eg, location), but also about its owner. Since smartphones handle a huge amount of private information, recent techniques rely on those sensing capabilities to authenticate the user, measuring her biometric features. In this paper, we survey the most relevant biometric authentication methods on smartphones proposed in the literature. We categorize such methods by the nature of biometrics used, by their temporal features and by the sensing capabilities they rely on. Moreover, we draw some future directions in this promising research topic.