Lectins are naturally occurring, carbohydrate-binding molecules that can be isolated from diverse biological sources and used in the laboratory to investigate the presence of carbohydrate structures in or on cells, in much the same way as antibodies can be used to probe cells and tissues for the presence of specific antigens. As it is becoming increasingly apparent that subtle alterations in the glycosylation of cancer cells can profoundly influence their biological behavior (with consequent implications for patient outcome and prognosis), lectin histochemistry is a potentially useful modification of the more widely used technique of immunohistochemistry. This chapter provides an introductory background to lectins and their use in breast cancer research, and provides basic protocols for lectin histochemistry that highlight the important technical differences between this approach and immunohistochemistry. The methods given here are broadly applicable and can be modified to investigate virtually any glycosylation change of potential interest in breast cancer research.