Mannan, as compounds of plant biomass, seem to be a very useful feedstock for the effective utilization in biotechnology. This is due to their abundance and renewability as well as the possibility of producing various chemical by-products, fuels, fodder and food products from this source. On the other hand, the enzymatic removal of mannans from cellulosic pulps and coffee extracts by mannanases is very important for the production of high quality paper and instant coffee. This review article presents new trends in the investigations on plant mannans and their enzymatic hydrolysis by mannanases of different origin. Particular emphasis has been laid on catalytic properties and molecular structure of different enzymes of mannanolytic complex, and on the mechanisms of their regulation and adsorption on wood fibers as well as on genes encoding particular enzymes. Most promising prospects for the practical applications of mannanases are also discussed.