A recent surge in newly described inborn errors of immune function-related genes that result in susceptibility to fungal disease has greatly enhanced our understanding of the cellular and molecular basis of antifungal immune responses. Characterization of single-gene defects that predispose to various combinations of superficial and deep-seated infections caused by yeasts, molds, and dimorphic fungi has unmasked the critical role of novel molecules and signaling pathways in mucosal and systemic antifungal host defense. These experiments of nature offer a unique opportunity for developing new knowledge in immunological research and form the foundation for devising immune-based therapeutic approaches for patients infected with fungal pathogens.