The remarkable complexity of oligosaccharide structures found on vertebrate cells results from the concerted action of glycosyltransferase enzymes (Joziasse, 1992; Stanley and Ioffe, 1995; Varki and Marth, 1995; Whitfield and Douglas, 1996), several of which were first identified and characterized by Robert Hill (Paulson et al, 1977; Paulson et al, 1978; Beyer et al, 1979; Rearick et al, 1979; Sadler et ai, 1979, 1982). Together with the structural characterization of naturally occurring oligosaccharides, the study of these enzymes has led to the definition of the major structural motifs of vertebrate sugar chains (Cummings, 1992; Kobata and Takasaki, 1992; Varki and Freeze, 1994; Varki and Marth, 1995). These motifs can be divided into (1) the core regions unique for each class of glycoconjugate (e.g., the Chitobiosyl-N-Asn-linkage region); and (2) the outer chains (e.g., type 1 and 2 lactosamine units with sialylation and/or fucosylation) that can be shared to varying extents by the different core classes (Cummings, 1992; Kobata and Takasaki, 1992; Varki and Freeze, 1994; Varki and Marth, 1995). Vertebrate cells utilize a relatively limited subset of the monosaccharides known to exist in nature, and in only some of the many possible combinations. However, several unusual variations of the typical structures exist, as well as a variety of specific modifications of the individual monosaccharide units. Hypothesizing that such subtle variations and modifications are more likely to mediate specific biological functions (Varki, 1993), we have paid special attention to their existence, structure and biosynthesis. Here I briefly describe some examples that my group has studied over the past decade.