Immune tolerance is a known immune cascade of events by which our immune system can regulate its function, avoiding unwanted immune response reactions to immune privilege sites in our body. The role of HLA-G in fetal-maternal immune tolerance to prevent the embryo from being rejected can be applied to the process of transplantation as well as other clinical applications. The gut is also an important site of immune tolerance, with the constant assault of food antigens and its billions of resident microbes. In transplantation, the level of expression of HLA-G in the graft tissues correlates with organ acceptance and controls the recipient's immune response. Furthermore, tumor immune escape is associated with both the expression of immune checkpoint molecules on peripheral immune cells and soluble forms of the human leukocyte antigen-G (HLA-G) in the blood, which is consequently discussed as a clinical biomarker for disease status and outcome of cancer patients. Future studies are needed to explore more immune tolerance pathways for HLA-G and to apply and use this in transplantation to prevent rejection and treat miscarriage cases and autoimmune diseases. In addition, therapies to block HLA-G in malignant diseases are exciting and need more clinical trials. This chapter addresses and reviews the published articles related to the advances in HLA G and immune tolerance.