Protein dimers are common in catalysis and regulation. Their associations are either homodimers (identical monomers) or heterodimers (nonidentical monomers). The molecular principles of protein dimer interactions are difficult to understand mainly due to the geometrical and chemical characteristics of proteins. Nonetheless, the principles of protein dimer interactions are often studied using a dataset of 3D structural complexes determined by X-ray crystallography. A number of physical and chemical properties govern protein dimer interactions. Yet, a handful of such properties are known to dominate protein dimer interfaces. The differences between homodimer and heterodimer interfaces using a selected set of interface properties are discussed.