T cell immunoglobulin-3 (Tim-3) has been identified as a marker of differentiated interferon-γ-producing CD4⁺ T helper type 1 and CD8⁺ T cytotoxic type 1 cells. The interaction of Tim-3 with its ligand, galectin-9 (Gal-9), induces cell death, and in vivo blockade of this interaction results in exacerbated autoimmunity and abrogation of tolerance in experimental models, establishing Tim-3 as a negative regulatory molecule. Recent studies have uncovered additional mechanisms by which Tim-3 negatively regulates T cell responses, such as promoting the development of CD8⁺ T cell exhaustion and inducing expansion of myeloid-derived suppressor cells. In contrast to this inhibitory effect on T cells, Tim-3–Gal-9 interaction promotes macrophage clearance of intracellular pathogens. Here, we focus on the emerging role for Tim-3 in tumor and antimicrobial immunity.