Unexpectedly, many cancers appear to induce a spontaneous adaptive T cell response. The presence of a T cell infiltrate has been linked to favorable clinical outcome in multiple cancer types. However, the innate immune pathways that bridge to an adaptive immune response under sterile conditions are poorly understood. Recent data have indicated that tumors can induce type I interferon (IFN) production by host antigen-presenting cells (APCs), which is required for a spontaneous T cell response in vivo. The innate immune sensing pathways that trigger type I IFN production are being elucidated. Host type I IFNs are also required for optimal therapeutic efficacy with radiation. This recently uncovered role for host type I IFNs for antitumor immunity has important fundamental and clinical implications.