Galectins are a family of animal lectins with conserved carbohydrate-recognition domains for b-galactoside. Galectin-3 is the only family member that is composed of a glycine/proline-rich N-terminal repeated sequence and a C-terminal carbohydrate-binding domain. Multiple functions of galectin-3 have been reported, depending on its location. Extracelluar galectin-3 can bind to cell surface through glycosylated proteins and thereby trigger or modulate cellular responses such as mediator release or apoptosis. Intracellular galectin-3 has been reported to inhibit apoptosis, regulate the cell cycle, and participate in the nuclear splicing of pre-mRNA. Recent studies have revealed that galectin-3 is expressed in a variety of cell types in the immune system, constitutively or in response to microbial invasion. These studies implicate galectin-3 in both innate and adaptive immune responses, where it participates in the activation or differentiation of immune cells. This review summarizes the roles of galectin-3 in the immune system and discusses the possible underlying mechanisms.