To form metastases, cancer cells must leave the immunosuppressive tumour microenvironment and traffic, predominantly in the circulation, to new tissue sites, where they must then expand. During this process, the tumour cells are open to attack by the immune system. This Review highlights the possible mechanisms used by circulating tumour cells in the blood and disseminated tumour cells in other tissues to evade, escape, or subvert the immune system in order to survive and form metastatic lesions.