The genetic alterations acquired by cancer cells are identified by diverse immune mechanisms, creating a complex network of interactions that can either favor or control tumor growth. Defects and impairments in the immune system are associated with cancer development. Compelling new evidences are also available regarding the protective value of anti-tumor adaptive immune responses, both local and systemic, developed by the host. More recently, the identification of new subsets of T helper, T cytotoxic, and dendritic cells, unraveled new forms of interactions between immune and tumor cells. The immune system is a powerful ally in the control of cancer development, metastasis and recurrence, due to two important properties that are absent in most anti-cancer treatments – specificity, and long-lasting memory. These properties are being increasingly explored in cancer therapy, from the wide use of monoclonal antibodies to the still experimental dendritic cell based therapies. Now, more than ever, the preservation as well as the recruitment of immune responses in the host constitute important approaches to be applied in cancer therapy.