Glycan parts of glycoconjugates on the surfaces of cells regulate many kinds of interactions between the cells and their immediate environments. Alterations in glycosylation on the cancer-associated glycoproteins are responsible for changes in their molecular interactions and biological functions. Glycosylation changes occur in the core and/or at the nonreducing end of the oligosaccharide chains of N-glycans. In this review, we focus on the branching of the common core structure of N-glycans, the responsible enzyme, and the extensions of some of the branches causing size increases on the surface of tumor cells. Abnormal branching, elongation of the branches, and increasing size of the common core of N-glycans are the typical features of these changes and are related with malignant transformations. Seven N-acetylglucosaminyltransferases (GnTs) (GnT-I, GnT-II, GnT-III, GnT-IV, GnT-V, GnT-VI, and GnT-IX) and a1,6-fucosyltransferase (FUT8) initiate the new branches on the core. GnT-IV, GnT-V, and GnT-IX initiate the branches available for poly-LacNAc extensions, which are responsible for tumor progression and metastasis. GnT-III prevents the catalytic activity of GnT-II, GnT-IV, GnT-V, and FUT8 to form branching and elongation of the branches. The contributions of GnT-III and the other enzymes to the cancer progression are in conflict with each other. While GnT-III prevents cancer, the others increase metastasis. The function of FUT8 is related to signal transduction and its activity is higher in tumor tissue than in healthy tissue. The impact of glycosylation changes on some of the cancer-associated proteins (growth factor receptors, adhesion and signal molecules, CD147, TIMP-1, and matriptase) is also summarized.