More than half of non-small cell lung cancer (NSCLC) patients are at an advanced stage at the time of diagnosis, and they have a poor prognosis. Systemic treatment is the basic treatment approach for advanced-stage NSCLC, and chemotherapy and targeted treatments are commonly used based on the molecular characteristics. Although targeted therapies have led to a significant level of improvement in terms of survival, the results are still unsatisfactory. However, considerable attention has been focused to the immunotherapy with recent positive results reported by studies on this field. In this context, a certain portion of clinical studies have shown dramatic results, and these have involved inhibitors developed particularly against the immune checkpoint protein programmed death receptor-1 and its ligand (programmed death ligand-1). This review aims to present the significance of immune checkpoint inhibitors in NSCLC and to summarize the findings of relevant contemporary clinical studies.