Glycosynthases are engineered glycoside hydrolases that in suitable reaction conditions promote the synthesis of oligosaccharides with exquisite stereoselectivity and enhanced regioselectivity, if compared to traditional chemical methods. This approach was demonstrated to be successful in a number of cases including β-glycosynthases acting at the termini or within an oligosaccharide chain (exo- and endo-glycosynthases, respectively) and, more recently, α-glycosynthases. This led to the production of a vast repertoire of products that include poly- and oligosaccharides, glycoconjugates, and glycopeptides. These molecules can be used as ligands of glycoside hydrolases, for the characterization of therapeutic enzymes, and as leads of drugs for the pharmaceutical industry. In this panorama, hyperthermophilic organisms, which thrive at temperatures as high as 80°C, which usually impede the growth of other living forms, have been used in the development of interesting novel glycosynthases. In fact, the extreme stability of these catalysts to extremes of pH and high concentrations of organics has allowed the exploration of novel reaction conditions, revealing new avenues for enzyme-catalyzed oligosaccharide synthesis.