In this present work we will highlight the multitude use of in vitro methods as routine technique in the: i) determination of the nutritive value of feed stuff (Mediterranean shrub species for ruminants coupled with its chemical composition, ii) detection of tannins in shrub species by using polyethylene glycol (PEG), and iii) determination of interspecies differences between sheep and goats capacity to use tanninferous species. Correlations between in vitro digestibility, kinetics of gas production and chemical composition are also discussed. Comparison of this technique with in sacco is reviewed.