The outcome of chemotherapy can be influenced by the host immune system at multiple levels. Chemotherapy can kill cancer cells by causing them to elicit an immune response or alternatively, by increasing their susceptibility to immune attack. In addition, chemotherapy can stimulate anticancer immune effectors either in a direct fashion or by subverting immunosuppressive mechanisms. Beyond cancer-cell-intrinsic factors that determine the cytotoxic or cytostatic response, as well as the potential immunogenicity of tumor cells, the functional state of the host immune system has a major prognostic and predictive impact on the fate of cancer patients treated with conventional or targeted chemotherapies. In this Review, we surmise that immune-relevant biomarkers may guide personalized therapeutic interventions including compensatory measures to restore or improve anticancer immune responses.