Chicken IL-18 (Interleukin-18) is a pro-inflammatory cytokine belonging to the IL-1 family that plays an important role in innate and adaptive immune responses in chickens (Gallus gallus). IL-18 is produced primarily by macrophages, dendritic cells, and epithelial cells as an inactive precursor that requires cleavage by caspase-1 following inflammasome activation to become biologically active. Once activated, IL-18 stimulates production of interferon-γ (IFN-γ) by T cells and natural killer (NK)-like cells, thereby promoting Th1-type immune responses and enhancing cell-mediated immunity against intracellular pathogens. In healthy chickens, basal IL-18 expression is generally low, but levels increase during viral, bacterial, and parasitic infections, including Marek’s disease virus (MDV), infectious bursal disease virus (IBDV), Salmonella spp., and Eimeria spp. IL-18 has also been studied as a molecular adjuvant to enhance vaccine-induced cellular immunity in poultry. In avian immunology and poultry research, chicken IL-18 serves as a biomarker of inflammasome activation and Th1-skewed immune responses, providing insight into host–pathogen interactions, vaccine efficacy, and immune regulation in commercial and experimental poultry systems.