Human leukocyte antigen-G (HLA-G) is a non-classical HLA class I molecule, which was first discovered in 1987 by Geraghty and colleagues. While classical HLA class I molecules are expressed on all nucleated cells, the expression of the HLA-G molecule is highly tissue-restricted, such as to placental trophoblast cells. HLA-G binds inhibitory receptors such as leukocyte immunoglobulin-like receptors B1 (LILRB1/ILT2/CD85j) and LILRB2 (ILT4/CD85d), which are widely expressed on immune cells, to suppress a broad range of immune responses. Thus, the expression of HLA-G in placenta protects the fetus from the maternal immune system. On the other hand, emerging studies have shown the relevance of the HLA-G molecule in pathologic conditions, such as transplantation rejection, autoimmunity, and cancer. HLA-G has other unique characteristics, in contrast with classical HLA molecules, including the existence of various forms of HLA-G: several splice variants, subunit-deficient conformations, homodimers, and their combinations have been found. In this review, we highlight the molecular basis for the tolerogenic ability of the HLA-G molecule, especially by LILR recognition of various forms of HLA-G. We also discuss the potential clinical applications of HLA-G molecules.