Detailed knowledge of the immunologic properties of embryonic stem (ES) cells is a prerequisite for safe applications of ES cell-based regenerative medicine. Recently, the long-standing assumption that ES cells are ignored by immunocompetent hosts was disproved. Instead, it is becoming increasingly clear that ES cells actively protect themselves via several immunomodulatory and immunoevasive mechanisms against cytotoxic T-lymphocytes and natural killer cells. Here we review current knowledge about the immunologic properties of ES cells and discuss the implications for ES cell-based regenerative medicine, for the immunobiology of the embryo as a semi-allogeneic graft, and for the regenerative capacity of adult stem cells.