Indoor photovoltaic cells have the potential to power the Internet of Things ecosystem, including distributed and remote sensors, actuators, and communications devices. As the power required to operate these devices continues to decrease, the type and number of nodes that can now be persistently powered by indoor photovoltaic cells is rapidly growing. This will drive significant growth in the demand for indoor photovoltaics, creating a large alternative market for existing and novel photovoltaic technologies. With the re-emergence of interest in indoor photovoltaic cells, we provide an overview of this burgeoning field focusing on the technical challenges that remain to create energy autonomous sensors at viable price points, and the commercial challenges to be overcome for individual photovoltaic technologies to dominate this market. I. INTRODUCTION TO INDOOR PHOTOVOLTAICS The early years of solar-power electronic devices saw photovoltaic (PV) cells used in extremely low power but relatively expensive consumer devices, where their cost could easily be absorbed. For example, the designers of a smartwatch that sold for ~$100 could afford to incorporate a cell costing a few dollars. Figure 1 outlines how, as the number and types of low-power electronic devices have expanded over the years, their costs have reduced. At the same time, the cost of PV cells has also been reducing, and the performance increasing, so that now we can sensibly power a range of electronic devices including wireless sensors, RFID tags or Bluetooth Beacons. A significant portion of these new devices are part of the Internet of Things (IoT) ecosystem that promises large networks of connected devices collecting the Big Data upon which our medical, manufacturing, infrastructure, and energy industries will be monitored and optimized. Billions of wireless sensors are expected to be installed over the coming decade, with almost half to be located inside buildings [1]. Currently, the use of batteries to power these devices places significant constraints on their power consumption, where the range and frequency Technology and market perspective for indoor photovoltaic cells