The industrial Internet-of-things connects machines, vehicles, plants and processes to optimize their performance, efficiency and safety. The emergence of AI provides even bigger opportunities but requires large amounts of data. Thus, wireless sensors to collect, preprocess and transmit data are a mandatory component of the IoT. Ideally, these sensors are maintenance-free and easy to install, requiring no access to the power grid. The technology of energy harvesting is the key for self-powered, wireless IoT sensors together with strict power management and ultra-low power circuit technologies. The presentation introduces different implementations of self-powered wireless sensors including a characterization of their performance and explains typical use cases.