Galectins comprise a group of animal lectins characterized by their specificity to β-galactosides and an evolutionarily conserved sequence motif in their carbohydrate-binding domain. Based on molecular structure, galectins are divided into three types: prototype, chimera-type, and tandem-repeat-type [1] (Fig. 77.1). They are expressed throughout the body and function both intracellularly and extracellularly, playing roles in various life phenomena, including immune regulation, cell differentiation, and signal transduction. Galectins have been implicated in a wide range of diseases, such as cancer, infectious diseases, and inflammation. Galectins bind to certain bacteria and kill them or are involved in the breakdown of bacteria that have invaded host cells. In addition, increased serum galectin-3 levels have been reported in various cancers. Furthermore, concentrations of free galectin-2, -3, -4, and -8 in the blood of patients with colon and breast cancer, especially those with metastases, have been reported to be significantly increased [2]. Clinical trials are currently being conducted for some galectin inhibitors as therapeutic drugs for cancer [3].