The mechanism of the water-gas shift reaction is discussed for both copper/zinc oxide/alumina and iron oxide/chromium oxide catalysts. The associative and regenerative mechanisms are presented and the evidence concerning each mechanism is critically reviewed. It is concluded that for the low temperature shift reaction over copper/zinc oxide/alumina catalysts considerable evidence exists to support both mechanisms and it is possible that either could proceed on the catalyst surface. For the iron oxide/chromium oxide catalysed high temperature shift reaction the experimental evidence supports a regenerative mechanism.