Tannins have long been thought of as antifeedants owing to their presumed digestibility-reducing properties. In this thesis information, at the molecular level, is presented in a reassessment of this assumption and the apparency theory of plant chemical defence which is dependent upon it. An introductory review provides chemical and opperational descriptions of tannins and a general outline of their ecological impact. Detailed attention is given to (i) tannin-protein complex formation and (ii), an assesment of in vivo evidence concerning the effects of tannins on herbivores. It is concluded that the evidence does not support the hypothesis that tannins uniformly reduce digestion, even though they do generally act as antifeedants. A series of crude tannin-containing plant extracts were prepared and characterised by chemical analyses and by their ability to precipitate protein and inhibit pepsin/cellulase digestion of celulose. Results indicated between-tannin variation but not that the chemical properties of crude tannins might be used to predict their interaction with the other components of a herbivore's diet. Experiments under conditions where soluble tannin-protein complexes formed and which modelled some digestive systems, showed that tannins could under varying circumstances, inhibit or promote the digestion of protein. Soluble tannin-protein complexes were also formed in the presence of bile salts when they would otherwise have occured as precipitates. In these conditions clear relief from digestibility reduction was found. In the light of these results a new model describing the effects of tannins on digestion, consistent with results obtained in vivo, is proposed.