The relationships between cytokines and cancer are multiple and bidirectional. On the one hand, cytokines may directly influence carcinogenesis and metastasis by modifying the tumor phenotype. On the other hand, during tumor progression, modifications of the cytokine expression in the tumor environment may be induced by the tumor cells, leading to a state of immunosuppression reflected by low cytokine expression in tumor stroma. Cytokines also play a role by stimulating the host immune system to generate anti-tumor specific responses. Finally, the use of cytokines as anti-tumor agents has led to objective clinical responses in about 15-25% of patients with metastatic melanoma or renal cell carcinoma, which presents the basis for the development of promising immunotherapeutic approaches for cancer therapy.