This chapter presents a detailed study of free-space optical (FSO) communication systems, emphasizing their role in next-generation wireless networks. FSO technology operates in the unlicensed optical spectrum, offering high-speed data transmission, minimal latency, and enhanced security. However, its performance is impacted by atmospheric turbulence, misalignment errors, and weather conditions. The chapter explores various aspects of FSO communication, including system architecture, channel modeling, and modulation techniques. It also examines advanced performance enhancement methods such as multiple-input multiple-output (MIMO) schemes, cooperative relaying, hybrid FSO/RF communication, and reconfigurable intelligent surfaces (RIS). Additionally, the integration of FSO with non-terrestrial networks (NTNs), including high-altitude platform stations (HAPS) and unmanned aerial vehicles (UAVs), is discussed for global connectivity solutions. In a nutshell, this chapter highlights FSO communication as a promising solution for 6G and beyond, addressing critical challenges in high-speed, secure, and energy-efficient wireless networks.