Originally the only known heterodimeric cytokine, IL-12 is now part of a family of five cytokines and shares important functions in the regulation of both innate and adaptive immunity with two of them, IL-23 and IL-27. Although initially these three cytokines were considered to have largely overlapping immunological functions, more recent studies, including two articles in this issue of ImmunityHamano et al. 2003, Villarino et al. 2003, indicate that they mediate complex and well-differentiated functions.