Galectin’s control diverse cellular functions such as cell death, angiogenesis, cancer cell metastasis and tumor-immune escape. Increased levels of galectin-3 (gal-3) in patients have been associated with cancer progression, and tumor-derived gal-3 is also used in diagnosis. For example, in some cancers increased expression of gal-3 within cells is a marker of the advanced phases of progression, with high levels of galectin-3 in the blood correlating with tumour metastasis. High galectin levels have also been shown to interfere with the effectiveness of cancer immunotherapy, including checkpoint inhibitors. Therefore, blocking galectins such as gal-3 is being investigated as a potentially novel approach to treating some types of cancer.