While nucleic acid binding species (aptamers) have previously been selected against a variety of biomedically important protein targets, the selection of aptamers against complex targets, such as cell surfaces and organisms, is becoming increasingly feasible. A number of aptamers have now been generated that can target specific cell-surface antigens and cell types, including tumor biomarkers and tumor cells. The generation of anti-cell aptamers has important implications for identifying disease-specific biomarkers, generating diagnostics, and developing novel drug delivery strategies.