**9. Summary**

Microbial xylanases are gaining importance in industrial applications. The thermophilic microorganisms are of interest in the field of biotechnology because of their ability to express enzymes that tolerate high temperature and pH. The thermophilic bacteria, and an archaeal species, there are also filamentous fungi that grow in diverse environments and produce xylanase. New recombinant DNA technology can be used to express xylanases in both homologous and heterologous host organisms. Compared to *E. coli, P. pastoris* considered the most suitable and excellent host for the heterologous proteins expression including xylanases. Xylanases generally must be genetically modified in order to increase the expression level with improved xylanase activity, but these alterations can change substrate specificity and stability at high temperatures and pH. Many xylanases have xylan-binding domains that can affect the thermostability of the enzymes and their ability to bind insoluble substrates. In conclusion, the lignocellulosic biomass and enzymeproducing microorganisms are compelling subjects for research studies aiming to explore the growing importance of xylanases in various industrial products.
