Human IL-18 (Interleukin-18) is a pro-inflammatory cytokine belonging to the IL-1 family that plays a central role in innate and adaptive immune responses. IL-18 is produced primarily by monocytes, macrophages, dendritic cells, epithelial cells, and other tissue-resident cells as an inactive precursor that requires cleavage by caspase-1 following inflammasome activation to become biologically active. Once released, IL-18 stimulates production of interferon-γ (IFN-γ) by T cells and natural killer (NK) cells, thereby promoting Th1-type immune responses and enhancing cell-mediated immunity against intracellular pathogens. IL-18 can also contribute to Th2 or Th17 responses depending on the cytokine milieu. In healthy individuals, circulating IL-18 levels are generally low but detectable; concentrations increase during infection, inflammatory stimulation, and immune activation. Elevated IL-18 has been implicated in autoimmune and inflammatory diseases such as rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, psoriasis, and adult-onset Still’s disease, as well as in sepsis, metabolic syndrome, and certain malignancies. In clinical and translational research, human IL-18 serves as a biomarker of inflammasome activation and immune dysregulation and is being investigated as both a therapeutic target and, in modified forms, as an immunostimulatory agent in cancer immunotherapy.