The PD-1 checkpoint pathway, consisting of the receptor PD-1 and its ligands PD-L1 and PD-L2, plays a critical role in maintaining peripheral tolerance by suppressing T-cell activation. Tumors and chronic pathogens exploit this pathway to evade immune responses. Therapies targeting PD-1 and its ligands have revolutionized cancer treatment by enhancing antitumor immunity. PD-1 is a 288-amino acid protein expressed on activated T cells and other immune cells. Its expression is regulated by transcription factors such as NFATc1, FOXO1, Notch, and IRF9, while T-bet represses PD-1 expression. PD-1 ligation recruits SHP-2, leading to the inhibition of T-cell signaling pathways such as PI3K-Akt and Ras-MEK-ERK, which results in T-cell quiescence and immune suppression. PD-1 also alters T-cell metabolism by inhibiting glycolysis and promoting fatty acid oxidation, which can impair T-cell function. PD-L1 and PD-L2 are expressed on various cell types, including antigen-presenting cells and cancer cells, and their interaction with PD-1 contributes to immune evasion. The PD-1 pathway is involved in both immune tolerance and T-cell exhaustion, with PD-1 blockade enhancing antitumor immune responses. However, PD-1 inhibition can also lead to autoimmune-like side effects due to the loss of immune regulation. The clinical success of PD-1/PD-L1 inhibitors is influenced by factors such as tumor mutational burden and the presence of neoantigens. Understanding the molecular mechanisms of PD-1 signaling is crucial for optimizing immunotherapy and identifying biomarkers for patient selection. The PD-1 pathway's role in cancer immunotherapy highlights the importance of balancing immune activation and tolerance to achieve effective treatment outcomes.The PD-1 checkpoint pathway, consisting of the receptor PD-1 and its ligands PD-L1 and PD-L2, plays a critical role in maintaining peripheral tolerance by suppressing T-cell activation. Tumors and chronic pathogens exploit this pathway to evade immune responses. Therapies targeting PD-1 and its ligands have revolutionized cancer treatment by enhancing antitumor immunity. PD-1 is a 288-amino acid protein expressed on activated T cells and other immune cells. Its expression is regulated by transcription factors such as NFATc1, FOXO1, Notch, and IRF9, while T-bet represses PD-1 expression. PD-1 ligation recruits SHP-2, leading to the inhibition of T-cell signaling pathways such as PI3K-Akt and Ras-MEK-ERK, which results in T-cell quiescence and immune suppression. PD-1 also alters T-cell metabolism by inhibiting glycolysis and promoting fatty acid oxidation, which can impair T-cell function. PD-L1 and PD-L2 are expressed on various cell types, including antigen-presenting cells and cancer cells, and their interaction with PD-1 contributes to immune evasion. The PD-1 pathway is involved in both immune tolerance and T-cell exhaustion, with PD-1 blockade enhancing antitumor immune responses. However, PD-1 inhibition can also lead to autoimmune-like side effects due to the loss of immune regulation. The clinical success of PD-1/PD-L1 inhibitors is influenced by factors such as tumor mutational burden and the presence of neoantigens. Understanding the molecular mechanisms of PD-1 signaling is crucial for optimizing immunotherapy and identifying biomarkers for patient selection. The PD-1 pathway's role in cancer immunotherapy highlights the importance of balancing immune activation and tolerance to achieve effective treatment outcomes.