Cancer evades the ability of the immune system by turning on cell surface proteins which turn off immune-surveillance cells such as T-helper cells. Immunotherapy treatments such as the immune checkpoint inhibitors overcome cancer’s ability to switch off the immune response to altered cells. Randomised trials studying immunotherapy have demonstrated a patient benefit across a number of disease sites, in both curative and non-curative settings.Pembrolizumab, a monoclonal antibody for the PD-1 receptor (B cells and T cells), has been NICE-approved in 2017 for the treatment of metastatic non-small-cell lung cancer (NSCLC) in second-line therapy and is available via the cancer drugs fund in England for first line therapy. Immune checkpoints such as the PD-1 receptor down-modulate the immune response as described above. The KEYNOTE-010 trial showed that pembrolizumab was better than standard second-line chemotherapy in terms of disease progression and toxicity. The particularly impressive outcome from many such immunotherapy studies is controlled or absent disease 5 years after starting treatment in a substantial cohort of patients.Adverse effects from immune checkpoint inhibitors are largely related to overstimulation of the immune system and include pneumonitis, hepatitis, endocrinopathy, skin rashes and gastrointestinal toxicity. There is growing evidence that radiotherapy delivered before or with immunotherapy, increases the likelihood a clinical response, and further investigations are under way.