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Immuno-oncology and in particular checkpoint inhibitor (CPI) treatment has become a novel promising cancer therapy strategy in recent years. However, still a minority of patients respond to checkpoint blockade. Primary and secondary resistance to CPI is a challenge in the daily clinical routine. Combination strategies have been tested in various clinical trials in order to address this issue. Data available from these trials indicate improved activity depending on the tumor type. This review article focuses on the molecular background for resistance to CPI, gives an overview of current clinical data of CPI combination studies and points out potential strategies to overcome CPI resistance depending on the immune phenotype.