This chapter provides a comprehensive overview of immune checkpoint molecules, focusing on their structure, function, and expression patterns. It begins by introducing the concept of immune checkpoints as critical regulators of immune responses, maintaining the delicate balance between effective pathogen clearance and prevention of autoimmunity. The chapter then explores detailed descriptions of key checkpoint molecules, including CTLA-4, PD-1, TIM-3, LAG-3, CD244, CD160, BTLA, TIGIT, and others. For each molecule, the chapter studies its structural components, signaling mechanisms, cellular expression, and known ligands. The role of these checkpoints in normal immune regulation, as well as their involvement in pathological conditions such as chronic infections and cancer, is thoroughly discussed. The chapter also covers emerging checkpoint molecules and their potential as therapeutic targets. Additionally, it examines the complex interplay between different checkpoint pathways and their impact on various immune cell populations, particularly T cells. The importance of these molecules in the development of cancer immunotherapies is highlighted, with discussions on the successes and challenges of checkpoint inhibition strategies. Throughout the chapter, it emphasizes the dual nature of many checkpoint molecules, capable of both inhibitory and stimulatory functions depending on the context to highlight the complexity of immune regulation. By providing this comprehensive review, the chapter aims to offer a solid foundation for understanding the current state of immune checkpoint research and its implications for future therapeutic developments.