Reconfigurable Intelligent Surfaces (RISs), also known as Intelligent Reflecting Surfaces (IRSs) or Large Intelligent Surfaces (LISs), have gained significant attention for their potential to enhance the capacity and coverage of wireless networks by smartly reconfiguring the wireless propagation environment. This paper provides a comprehensive overview of the state-of-the-art on RISs, focusing on their operating principles, performance evaluation, beamforming design, resource management, and applications of machine learning in RIS-enhanced wireless networks. The authors describe the basic principles of RISs from both physics and communications perspectives, present performance evaluations of multi-antenna assisted RIS systems, and survey existing designs for RIS-enhanced wireless networks. They also discuss the integration of RISs with other emerging technologies and identify major research opportunities. The paper highlights the advantages of RISs, such as ease of deployment, spectral efficiency enhancement, environmental friendliness, and compatibility with existing wireless network standards. It compares different categories of RISs, including metamaterial and patch-array based technologies, and discusses the operating principles of RISs, including waveguide, refraction, and reflection. The paper also explores the performance evaluation techniques for multi-antenna assisted RIS systems and the challenges in hardware implementation and optimization. Finally, it outlines the future research directions and potential applications of RISs in various wireless communication scenarios.Reconfigurable Intelligent Surfaces (RISs), also known as Intelligent Reflecting Surfaces (IRSs) or Large Intelligent Surfaces (LISs), have gained significant attention for their potential to enhance the capacity and coverage of wireless networks by smartly reconfiguring the wireless propagation environment. This paper provides a comprehensive overview of the state-of-the-art on RISs, focusing on their operating principles, performance evaluation, beamforming design, resource management, and applications of machine learning in RIS-enhanced wireless networks. The authors describe the basic principles of RISs from both physics and communications perspectives, present performance evaluations of multi-antenna assisted RIS systems, and survey existing designs for RIS-enhanced wireless networks. They also discuss the integration of RISs with other emerging technologies and identify major research opportunities. The paper highlights the advantages of RISs, such as ease of deployment, spectral efficiency enhancement, environmental friendliness, and compatibility with existing wireless network standards. It compares different categories of RISs, including metamaterial and patch-array based technologies, and discusses the operating principles of RISs, including waveguide, refraction, and reflection. The paper also explores the performance evaluation techniques for multi-antenna assisted RIS systems and the challenges in hardware implementation and optimization. Finally, it outlines the future research directions and potential applications of RISs in various wireless communication scenarios.