Work on the mechanisms of fetomaternal tolerance has undergone a renaissance in recent years, and the general outlines of a solution to this long-standing paradox of 'transplantation' immunology have come into view. Here, we discuss several mechanisms, recently described in mice, that either minimize the activation of maternal T cells with fetal or placental specificity, or minimize the possibility that such T cells, if activated, are able to harm the fetus. The T cell response to antigens expressed by the conceptus serves as a paradigm for the study of tissue-specific immune tolerance and is relevant to the pathogenesis of immune-mediated pregnancy complications.