Glycans mediate different functions: maintenance of cell structure, cell-cell and cell-extracellular matrix interactions, regulation of protein structural integrity and function, receptor signaling, and modulation of immune responses. In epithelial cancer cells, dysregulation of glycosylation leads to aberrant tumor-associated glycan or carbohydrate structures. Altered glycosylation promotes cell proliferation, adhesion, survival, migration, invasion, and trigger immune responses. In this review, we describe aberrant glycan features of cancer cells and glycosylation pathways. We also highlight glycosylation as a fundamental mechanistic concept associated with the hallmarks of cancer, focusing on the role of glycans in promoting genome instability, maintenance of cell proliferative signaling, evasion of growth suppressors, resistance to cell death, epithelial-to-mesenchymal transition, cell adhesion, migration and invasion, glucose metabolism, and angiogenesis, among other processes that promote cancer growth and metastasis.