Considerable progress is being made in our understanding of the molecular basis for mammalian carbohydrate recognition systems. Selectins, related proteins and sialoadhesins are carbohydrate-binding proteins which serve as receptors in the orchestration of innate and acquired immune responses, inflammation and other forms of cell-cell communication. Protein structural studies and gain-of-function and loss-of-function mutations are providing clues to ways in which the receptors interact with monosaccharide elements of the oligosaccharide ligands. Binding experiments using oligosaccharides on lipid or protein carriers indicate that modes of presentation such as the clustered state and the manner of display on proteins are crucial factors determining whether a functional triad of receptor and ligand + carrier (counter-receptor) is formed.