The interleukin-12 (IL-12) family, unique among cytokines for its heterodimeric structure, includes IL-12, IL-23, IL-27, and IL-35. Despite sharing structural and molecular features, these cytokines exhibit diverse functional effects. This review discusses the unique structural and functional characteristics of the IL-12 family, highlighting how cells interpret similar cytokine signals to produce diverse outcomes. The authors explore the implications of this complexity for therapeutic targeting and the potential for additional layers of regulation. They also discuss the role of chain sharing and receptor usage in shaping immune responses, emphasizing the balance between positive and negative regulators within the family. The IL-12 family's impact on T cell differentiation and disease outcomes is highlighted, with a focus on the immunoregulatory properties of IL-12 and IL-23 in promoting pro-inflammatory Th1 and Th17 cells, and IL-27 and IL-35 in inducing regulatory T cells. The review concludes by addressing future questions and challenges, including the potential for additional members, the physiological impact of chain sharing, and the complexity of signal interpretation.The interleukin-12 (IL-12) family, unique among cytokines for its heterodimeric structure, includes IL-12, IL-23, IL-27, and IL-35. Despite sharing structural and molecular features, these cytokines exhibit diverse functional effects. This review discusses the unique structural and functional characteristics of the IL-12 family, highlighting how cells interpret similar cytokine signals to produce diverse outcomes. The authors explore the implications of this complexity for therapeutic targeting and the potential for additional layers of regulation. They also discuss the role of chain sharing and receptor usage in shaping immune responses, emphasizing the balance between positive and negative regulators within the family. The IL-12 family's impact on T cell differentiation and disease outcomes is highlighted, with a focus on the immunoregulatory properties of IL-12 and IL-23 in promoting pro-inflammatory Th1 and Th17 cells, and IL-27 and IL-35 in inducing regulatory T cells. The review concludes by addressing future questions and challenges, including the potential for additional members, the physiological impact of chain sharing, and the complexity of signal interpretation.