AbstractProtein‐protein contacts in monomeric protein crystal structures have been analyzed and compared to the physiological protein‐protein contacts in oligomerization. A number of features differentiate the crystal‐packing contacts from the natural contacts occurring in multimeric proteins. The area of the protein surface patches involved in packing contacts is generally smaller and its amino acid composition is indistinguishable from that of the protein surface accessible to the solvent. The fraction of protein surface in crystal contacts is very variable and independent of the number of packing contacts. The thermal motion at the crystal packing interface is intermediate between that of the solvent‐accessible surface and that of the protein core, even for large packing interfaces, though the tendency is to be closer to that of the core. These results suggest that protein crystallization depends on random protein‐protein interactions, which have little in common with physiological protein‐protein recognition processes, and that the possibility of engineering macromolecular crystallization to improve crystal quality could be widened.