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 Immunity (Hamano et al., 2003; Villarino et al., 2003), indicate that they mediate complex and well-differentiated functions.