Metabolism of glycoproteins and glycolipids is accompanied by the appearance of unbound structural analogues of the carbohydrate portion of glycoconjugates or so called free oligosaccharides. There are their several sources inside the cell: 1) multistep pathways of N-glycosylation, 2) the cell quality control and ER-associated degradation of misglycosylated and/or misfolded glycoproteins, 3) lysosomal degradation of mature glycoconjugates. In this review the information about the ways of free oligosaccharides appearance in different cell compartments and details of their structures depending on the source is summarized. In addition, extracellular free oligosaccharides, their structures and changes under normal and pathological conditions are discussed.