Immune checkpoint inhibitors (ICIs)—such as antibodies to programmed cell death–1 (PD-1), to its ligand PD-L1, or to cytotoxic T lymphocyte-associated protein–4 (CTLA-4)—are an evolving treatment option for several types of cancer, but only a limited number of patients benefit from such therapy. Preclinical studies have suggested that the combination of PD-1 or PD-L1 inhibitors with either cytotoxic chemotherapy or antibodies to CTLA-4 is a promising treatment strategy for advanced cancer. Indeed, combinations of cytotoxic chemotherapy and PD-1/PD-L1 inhibitors have been approved and are now used in clinical practice for the treatment of advanced non-small cell lung cancer and small cell lung cancer on the basis of positive results of large-scale clinical trials. In addition, the combination of antibodies to CTLA-4 (ipilimumab) and to PD-1 (nivolumab) has been found to confer a survival benefit in patients with melanoma or renal cell carcinoma. Several ongoing clinical trials are also investigating ICI combination therapy in comparison with standard therapy for other tumor types. The identification of patients likely to achieve a sufficient benefit from PD-1/PD-L1 inhibitor monotherapy remains a challenge; however, with the establishment of novel complementary biomarkers being needed. Preclinical and clinical investigations of immune-related adverse events of ICI combination therapy are also warranted to establish management strategies. In this review, we summarize the current landscape of combination therapy with PD-1/PD-L1 inhibitors plus either cytotoxic chemotherapy or CTLA-4 inhibitors to clarify the benefits of and outstanding clinical issues related to such treatment.