Since their discovery as a distinct T helper (Th) cell lineage, Th17 cells have been extensively investigated both in mice and in humans. These studies have identified factors involved in their differentiation and effector functions and have also revealed a high degree of flexibility that seems to be a characteristic of the Th17-cell lineage. In this review, we discuss recent studies addressing the heterogeneity of human Th17 cells, their differentiation requirements, their migratory capacities, and their role in defense against fungi and extracellular bacteria.