Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting to detect proteins and glycoproteins is one of the most widely used and broadly useful techniques in cancer research, allowing the proteins in a complex sample--such as a blood sample, aspirate, or solid tumor homogenate--to be separated according to molecular weight and visualized within a gel matrix and/or, once separated, transferred onto a supporting membrane, where they may be probed for the binding of antibodies or lectins. In this chapter, the theory and principles of SDS-PAGE and Western blotting are briefly outlined, and basic methods are given that can be applied to investigate virtually any (glyco)protein of interest in breast cancer research.