Organic photovoltaics (OPVs) have shown great potential as a new generation of energy sources because they possess many unique properties, including mechanical flexibility, light weight, semitransparency, and low fabrication costs. In particular, OPV devices exhibit high power conversion efficiencies under indoor and low-level lighting conditions. Therefore, they can function as promising energy sources in low-light or cloudy environments for many applications, including the Internet of Things, wearable electronics, and sensors. In this article, we review recent progress in OPV devices for these special applications. We start with an introduction to the fundamental principles of OPVs. Then, we review the preparation and design principles of the photoactive layers for indoor applications. We also highlight the importance of interlayers in high-performance photovoltaic devices under indoor and/or low-level lighting illumination conditions. Recent efforts to improve the efficiencies of indoor OPV devices using plasmonic nanostructures are also summarized. Finally, we examine the progress in large-area devices and modules for indoor and/or low-level lighting applications. We believe that the rapid progress in indoor OPV cells and modules will trigger the development of low-cost, highly efficient OPV products for indoor applications in the near future.