The two main pregastric compartments of ruminants (rumen and reticulum) can be compared to an anaerobic fermenter which allows them to use energetic and nitrogen substrates which are indigestible by monogastric animals. The aim of this bibliographic review is to describe the main features (characteristics) of this fermenter, focusing on the role of microbes (bacteria, protozoa and fungi), on the degradation of cell-wall carbohydrates (cellulose, hemicelluloses, pectic substances), on nitrogen digestion and finally on the utilisation of the major minerals and trace elements.