The current data on microbial mannanases are summarised in the work. The questions concerning the spreading of mannanases substrates--mannans in nature, their structural variations are regarded. The production of mannanases by micro-organisms of different taxonomic groups, their physico-chemical properties and substrate specificity are described. The ways of mannans hydrolysis by mannanase are estimated. The questions of cloning and expression of mannanase genes have been discussed. These results are very important for producing highly effective microbial biosynthetics.