IL-23--produced by dendritic cells--and Th17 cells have both been identified as major factors involved in autoimmune inflammation, yet their relationship with each other remains controversial. This review aims to describe the initial discovery of Th17 cells, their subsequent characterization as a unique T helper subset in mouse and man, as well as the mechanisms involved in regulating these cells. Finally, the roles of IL-23 in inflammatory diseases in relation to Th17 function will be discussed.