While reasoners are year after year scaling up in the classical, time invariant domain of ontological knowledge, reasoning upon rapidly changing information has been neglected or forgotten. On the contrary, processing of data streams has been largely investigated and specialized Stream Database Management Systems exist. In this paper, by coupling reasoners with powerful, reactive, throughput-efficient stream management systems, we introduce the concept of Stream Reasoning. We expect future realization of such concept to have high impact on the future Internet because it enables reasoning in real time, at a throughput and with a reactivity not obtained in previous works.