Metastasis arise from solid tumors when the cancer cells undergo changes and progresses through a cascade that creates disseminated tumours. Metastasis is considered to be secondary tumours derived from a primary tumour. The fate of disseminated carcinoma cells is influenced by interaction during the transit through the circulatory system. The immune system can both promote and inhibit metastasis and it is of importance to know the mediators involved and how they impart their effect, in order to identify new targets to prevent metastatic disease.