AbstractThe origins and development of in‐vitro techniques for the determination of the apparent digestibility of ruminant feedstuffs are critically reviewed. Special attention is given to the effect of the pH used for the pepsin digestion stage, its optimum value, and its solubilising effect upon various mineral compounds and protein‐ and mineral‐tannin complexes formed in the ruminant digestive system. New experimental evidence is presented concerning the latter effect. Comparisons are made with the Tilley and Terry method in widespread use at the present time.