Most solid tumors contain hypoxic regions. Hypoxia affects a variety of tumor cell properties such as cell growth rate, neovascularization, metastasis and sensitivity to treatment. Breast 3cancer is the second most common cause of death in women. Nearly half of breast cancer patients treated for localized disease develop metastases and often combinations of local and systemic therapy are not curative. Tissue oxygenation measurements in human breast carcinomas have shown large areas of hypoxic tissue and immunolocalized signals of the hypoxic markers, CAIX and HIF-1 alpha, in breast cancer tissue show strong staining around necrotic regions. A wide range of genes associated with breast cancer metastasis have been reported to be upregulated under hypoxic conditions and hypoxic gene signatures are associated with poorer outcome in breast cancer. An understanding of the molecular pathways in hypoxia-induced breast cancer metastasis promises potential useful prognostic and therapeutic information.