Molecular mimicry has been touted as a mean to develop new generation of vaccines to target carbohydrate antigens on pathogens and on tumor cells. Structural and immunological rules governing molecular mimicry require definition for its successful exploitation. Of interest are the kinds of structures that peptides adopt as carbohydrate mimics, the extent to which topological or sequence similarities among peptide mimeotopes define serum cross-reactivity to carbohydrate antigens and the extent to which peptide mimeotopes affect T-cell responses. Rational design concepts can be applied to define how a peptide may mimic carbohydrate antigens, similarities in binding affinities of antibodies for carbohydrate and for peptides, how peptides can mimic core structures on otherwise dissimilar carbohydrate antigens, and how peptide mimeotopes can be used to manipulate cellular responses not achievable by carbohydrate antigens.