Mechanisms of transglycosylation reactions in the nucleoside chemistry and their modern interpretations are presented. Special attention is paid to the formation of kinetically controlled and thermodynamic products, reaction intermediates, correlation of tautomerism and regioselectivity in glycosyl migration reactions, their stereoselectivity and reversibility as well as side-reactions like anomerization or cyclization. Some synthetic applications of transglycosylation are also discussed.