The automatic and unobtrusive identification of user’s activities is one of the challenging goals of context-aware computing. This paper discusses and experimentally evaluates instance-based algorithms to infer user’s activities on the basis of data acquired from body-worn accelerometer sensors. We show that instance-based algorithms can classify simple and specific activities with high accuracy. In addition, due to their low requirements, we show how they can be implemented on severely resource-constrained devices. Finally, we propose mechanisms to take advantage of the temporal dimension of the signal, and to identify novel activities at run time.