This paper presents the development and validation of free-space path loss models for reconfigurable intelligent surfaces (RISs) in wireless communications. RISs, composed of tunable unit cells, can manipulate electromagnetic waves and offer significant performance improvements and coverage enhancement in a cost-effective and energy-efficient manner. The paper develops path loss models for RIS-assisted wireless communications by studying the physics and electromagnetic nature of RISs. These models are validated through extensive simulations and experimental measurements conducted in a microwave anechoic chamber using three fabricated RISs (metasurfaces). The results show that the proposed models accurately predict the path loss behavior of RIS-assisted wireless communications, taking into account factors such as the distances from the transmitter/receiver to the RIS, the size of the RIS, the near-field/far-field effects of the RIS, and the radiation patterns of antennas and unit cells. The paper also discusses the implications of these models for future research and practical applications in RIS-assisted wireless communications.This paper presents the development and validation of free-space path loss models for reconfigurable intelligent surfaces (RISs) in wireless communications. RISs, composed of tunable unit cells, can manipulate electromagnetic waves and offer significant performance improvements and coverage enhancement in a cost-effective and energy-efficient manner. The paper develops path loss models for RIS-assisted wireless communications by studying the physics and electromagnetic nature of RISs. These models are validated through extensive simulations and experimental measurements conducted in a microwave anechoic chamber using three fabricated RISs (metasurfaces). The results show that the proposed models accurately predict the path loss behavior of RIS-assisted wireless communications, taking into account factors such as the distances from the transmitter/receiver to the RIS, the size of the RIS, the near-field/far-field effects of the RIS, and the radiation patterns of antennas and unit cells. The paper also discusses the implications of these models for future research and practical applications in RIS-assisted wireless communications.