Tumor and host cells frequently express interleukin-8 (IL-8). IL-8 has been shown to be motogenic, mitogenic, and angiogenic and to play important roles in human tumor progression. IL-8 expression can be induced by numerous stress factors present in the tumor environment, such as hypoxia, acidosis, hyperglycemia, hyperosmotic pressure, high cell density, hyperthermia, radiation, and chemotherapeutic agents. Understanding the mechanisms of IL-8 expression and regulation will be helpful in designing potential therapeutic modalities targeting IL-8 to control tumor growth and metastasis.