Using a database of 6166 experimental structures taken from the Protein Data Bank, we have studied pair interactions between planar residues (Phe, Tyr, His, Arg, Glu and Asp) in proteins, known as pi-pi interactions. On the basis of appropriate coordinates defining the mutual arrangement of two residues, we have calculated 2-D potentials of mean force aimed at determining the stability of the most probable structures for aromatic-aromatic, aromatic-cation and aromatic-anion bound pairs. Our analysis reveals the thermodynamic relevance and the ubiquity of stacked complexes in proteins.