The coordination chemistry of mercury is an extremelybroad field, as shown by a survey covering the crystallographic and structural data of over 550 examples. About 12% of those complexes exist as isomers and are summarised and classified in this review. Included are distortion (73%), polymerisation (20.6%), coordination number (3.2%) and ligand (3.2%) isomerism. These are discussed in terms of the coordination around the mercury atoms, and correlations are drawn between donor atoms, bond distances and bond angles. Distortion isomers, differing only by the degree of distortion in the Hg — L and L — Hg — L angles, are the most common. These isomers are discussed and compared with those found in the chemistry of zinc and cadmium.