Reconfigurable Intelligent Surfaces (RISs) are emerging as a transformative technology in wireless communications, offering the ability to control the propagation environment to enhance signal quality. This article provides an overview of RISs, their operation, and their potential applications in 6G and beyond. RISs are man-made surfaces composed of electromagnetic materials that can be electronically controlled to shape the propagation of radio waves. Unlike traditional technologies such as relaying, MIMO beamforming, and backscatter communications, RISs are nearly passive, do not require dedicated energy sources, and can be easily deployed in various environments. They offer full-band response, can be reconfigured to adapt to changing wireless environments, and can significantly improve the received signal power by coherently combining reflected, refracted, and scattered waves. The article discusses the potential of RISs in enhancing wireless communication systems by controlling the propagation environment to overcome the limitations of natural wireless propagation. It highlights the fundamental differences between RISs and other technologies, the open research issues, and the need to rethink communication-theoretic models. The article also explores the theoretical performance limits of RIS-assisted communication systems and their potential use cases in 6G and beyond. It provides an illustrative example of how RISs can be used to control the propagation environment in a two-ray model, demonstrating the potential for significant improvements in signal quality and power efficiency. The article also discusses the error performance of RIS-assisted systems and the potential of RISs as low-complexity and energy-efficient transmitters. Overall, RISs represent a promising technology for future wireless networks, offering the potential to fundamentally change how wireless networks are designed and optimized.Reconfigurable Intelligent Surfaces (RISs) are emerging as a transformative technology in wireless communications, offering the ability to control the propagation environment to enhance signal quality. This article provides an overview of RISs, their operation, and their potential applications in 6G and beyond. RISs are man-made surfaces composed of electromagnetic materials that can be electronically controlled to shape the propagation of radio waves. Unlike traditional technologies such as relaying, MIMO beamforming, and backscatter communications, RISs are nearly passive, do not require dedicated energy sources, and can be easily deployed in various environments. They offer full-band response, can be reconfigured to adapt to changing wireless environments, and can significantly improve the received signal power by coherently combining reflected, refracted, and scattered waves. The article discusses the potential of RISs in enhancing wireless communication systems by controlling the propagation environment to overcome the limitations of natural wireless propagation. It highlights the fundamental differences between RISs and other technologies, the open research issues, and the need to rethink communication-theoretic models. The article also explores the theoretical performance limits of RIS-assisted communication systems and their potential use cases in 6G and beyond. It provides an illustrative example of how RISs can be used to control the propagation environment in a two-ray model, demonstrating the potential for significant improvements in signal quality and power efficiency. The article also discusses the error performance of RIS-assisted systems and the potential of RISs as low-complexity and energy-efficient transmitters. Overall, RISs represent a promising technology for future wireless networks, offering the potential to fundamentally change how wireless networks are designed and optimized.