In this paper, the evidence supporting two different models for the molecular mechanism of photoinhibition is discussed. One hypothesis centres around the suggestion that photoinhibition is due to the loss of the herbicide-binding Dl polypeptide of photosystem II. The other model suggests that damage to a functional group in the reaction centre is the primary cause of photoinhibition. In order to put the apparent controversy into context, recent developments in our understanding of the structure and function of the photosystem II reaction centre are described. Interpretation and judgement of all available evidence suggest primary photoinhibitory damage to be incurred by the reaction-centre chlorophyll P680 destabilising the apoprotein(s) and eventually resulting in their proteolytic degradation and removal from the photosystem II complex and the thylakoid membrane.