IL-15 has a pivotal role in life and death of natural killer (NK) and CD8 memory T cells. IL-15 signals through a heterotrimeric receptor involving the common gamma chain (γc) shared with IL-2, IL-4, IL-7, IL-9, and IL-21, IL-2/IL-15 receptor β (IL-15Rβ) shared with IL-2 and a private IL-15Rα subunit. IFN- or CD40 ligand-stimulated dendritic cells coordinately express IL-15 and IL-15Rα. Cell surface IL-15Rα presents IL-15 in trans to cells that express IL-2/IL-15Rβ and γc. IL-15 is being used to treat patients with metastatic malignancy. However, IL-15 is an inflammatory cytokine involved in immunological memory including that to self, thereby playing a role in autoimmune diseases. These insights provide the scientific basis for clinical strategies directed toward diminishing IL-15 action. Dysregulated IL-15 expression was demonstrated in patients with rheumatoid arthritis, inflammatory bowel disease, psoriasis, celiac disease, and alopecia areata. The monoclonal antibody Hu-Mik-β-1 targets the cytokine receptor subunit IL-2/IL-15Rβ (CD122), blocks IL-15 transpresentation, and is being used in clinical trials in patients with autoimmune diseases. In parallel, clinical trials have been initiated involving the Jak2/3 (Janus kinase-2/3) inhibitor tofacitinib and Jak1/2 inhibitor ruxolitinib to block IL-15 signaling.