In recent years, a breakthrough in tumor therapy was achieved with the development of checkpoint inhibitors. Checkpoint inhibitors activate the immune defense against tumors by overcoming the inhibitory effect of specific cell surface proteins acting as control points, the so-called checkpoints. This article provides an overview of the mode of action of approved checkpoint inhibitors and the status of current clinical development.The previously approved checkpoint inhibitors, monoclonal antibodies directed against the checkpoints CTLA‑4 and PD-1/PD-L1, are used in various tumor entities (including lung, kidney, and urothelial carcinoma; head and neck cancer; melanoma; and Hodgkin lymphoma). For the first time, long-term survival has been achieved in some of these patients with advanced tumors. Unfortunately, this efficacy can be observed only in a small proportion of the treated patients, depending on the tumor indication. Improved efficacy is envisioned by patient selection via predictive biomarkers and the development of combination therapies. Mandatory testing of the expression level of the predictive PD-L1 biomarker is already required in some indications to select patients with an enhanced benefit/risk relationship.