1. We have assessed, using current literature, the respiratory consequences of water-borne heavy metal exposure in crustaceans and molluscs. 2. We suggest that in lethal and sub-lethal concentrations the essential metals Cu and Zn act on the respiratory system primarily by disrupting gill function which results in the development of internal hypoxia, although reparation can be accomplished even at "high" sub-lethal concentrations. 3. The more toxic xenobiotes such as Hg (and perhaps to a lesser extent Cd) may interfere with the respiratory system at every level of organisation including cellular respiration itself.