Mercury is classed as a heavy metal both from its physical properties and its biochemical behaviour.This review includes over two hundred heterometallic mercury compounds of known crystal structure.The heterometal atoms include both transition and non-transition metals, including both hard (class A) and soft (class B) examples. The compounds range from simple hetero-double salts and hetero-bimetallic derivatives to heterometallic clusters of up to twenty one metal atoms. Examples of isomerism include distortion, polymer and linkage isomers. Correlations between structural parameters, heterometal and ligand donor atoms are developed and an overall summary of the metal-metal bonding distances is given.