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 evolutionary development of the immune response of vertebrates. A broad, many-faceted approach is taken toward fundamental issues in immunobiology in order to develop innovative concepts of receptor function as well as to delineate traditional views. The capacity of plants to discriminate between self and non-self is addressed in Chapter 1 by R. B. Knox and Adrienne E. Clarke. These authors provide examples of cell-cell recognition in plants that parallel those occurring in in­ vertebrates and vertebrates. In general, tolerance (acceptance) of grafts is re­ stricted to plants within closely related genera. Recognition is mediated by callus cells, which proliferate at wound surfaces in higher plants, and there is a correlation between cell and tissue type and antigenic markers detectable with the use of mammalian antibodies. Certain flowering plants exhibit precise discrimination in fertilization, when pollen must be from the same species, but fertilization occurs only if the pollen is genetically non-self.