The Internet of Things (IoT) is ushering a paradigm change in the concept of sensors. Humans are no longer the only users of data. More often than not, data generated by sensors is further used by automatic algorithms, which imposes higher standards for the actionability and interpretability of the data. Attaining these standards typically requires mathematical modeling, which is used to tie together multiple data sources and to learn the relevant information. This leads us to the concept of a virtual sensor.