Galectin-3 is a pleiotropic carbohydrate-binding protein involved in a variety of normal and pathological biological processes. Its carbohydrate-binding properties constitute the basis for cell-cell and cell-matrix interactions and cancer progression. Modulation of galectin-3 expression in cancer cells has indeed been reported. These observations lead to the recognition of galectin-3 as a diagnostic/prognostic marker for specific cancer types, such as thyroid and prostate. This review discusses the expression and cellular localization of galectin-3 in cancer cells, as well as its numerous functions in cancer cell biology, including cell-cell adhesion, cell-matrix interactions, growth regulation, apoptosis, angiogenesis and mRNA splicing.