Small-cell lung cancer (SCLC) is distinguished from non–small-cell lung cancer by its rapid growth and more frequent metastases. Although patients with SCLC are highly responsive to chemotherapy and radiation therapy, long-term prognosis remains poor, with relapse and disease recurrence occurring in almost all cases. Whereas combination chemotherapies continue to be the standard of care in extensive-stage SCLC, there is value in exploring whether immune-checkpoint inhibition is an effective treatment strategy, given the durable responses in non–small-cell lung cancer. Data from SCLC trials have shown clinical activity and response to cytotoxic T-lymphocyte antigen-4 protein and programmed cell death-1 blockade, suggesting that antibodies targeting these pathways may be effective in improving survival outcome. However, data on clinical activity by programmed cell death-1 ligand expression in SCLC are not widely available. Limited data indicate that programmed cell death-1 ligand expression may not be an ideal biomarker for patient selection. Continued research is necessary to better optimize patient selection and response to therapy.