The problem that virtually all cells have in discriminating between "self" and "non-self" molecules and cells has been considered at great length in immuno biology. However, cells that clearly are incapable of carrying out mammalian type immune functions can exhibit exquisite specificity in their capacity to discriminate among syngeneic, allogeneic, and xenogeneic cells. In this volume of Contemporary Topics in Immunobiology we have chosen to consider the general problem of self/non-self discrimination as it is manifest in recognition reactions of plants and invertebrates and in the...