The non-classical human leukocyte antigen (HLA) class I antigen HLA-G represents a tolerogenic molecule and is involved in the inhibition of natural killer cell and cytotoxic T lymphocyte-mediated cytotoxicity. Under physiological conditions, HLA-G expression is mainly restricted to immune-privileged tissues, whereas it is overexpressed in tumors and transplants as well as in virus-infected cells. Due to its immunosuppressive features, HLA-G is important for pregnancy or organ transplantation and autoimmune diseases as well as cancer immune escape. This review focusses on the expression, regulation, and function of HLA-G in tumor cells andlor in transplants as well as therapeutic tools for enhancing (transplantation) or avoiding (tumor) tolerance. Thus, HLA-G or HLA-G-derived synthetic molecules might be used as therapeutic agents in combination with immunosuppressive drugs to enhance organ tolerance upon transplantation. In addition, HLA-G neoexpressing tumor cells could be targeted by HLA-G-specific microRNAs in order to enhance tumor immunogenicity.