Although the potent effects of cation-p interactions for stabilizing protein structure and binding ligands to proteins have been reported in the chemical literature for over two decades, there are no undergraduate biochemistry textbooks covering this important noncovalent attractive force. In an attempt to remedy this situation, this primer describes what cation-p interactions are, provides biochemical examples of cation-p interactions that supplement the protein structure and ligand binding topics of a traditional one-semester biochemistry course, and offers exercises and worked out solutions on cation-p interactions derived from the biochemical literature. In addition, instructions are provided for using the online molecular visualization tools FirstGlance and JSmol viewer to identify cation-p interactions in proteins and for calculating cation-p stabilization energy with the online CaPTURE program.