The cation-pi interaction is a potent, general noncovalent binding force that is observed in a wide range of biological contexts. Here, we present an overview of well documented cases in which a cation-pi interaction makes an important contribution to small-molecule recognition at a protein binding site. From these and other studies it is clear that, in addition to the hydrophobic effect, hydrogen bonding and ion pairing, the cation-pi interaction must be considered when evaluating drug-receptor interactions.