Alterations in microRNA (miRNA) expression have been associated with tumor suppression or tumorigenesis, metastasis and poor prognosis in human breast cancer. Deregulation of miRNAs is emerging as a major aspect of cancer etiology because their capacity to direct the translation and stability of targeted transcripts can dramatically influence cellular physiology. Some miRNAs were considered to be associated with molecular subtypes of breast cancer and correlated with specific breast cancer clinicopathological factors, such as HER2, estrogen and progesterone receptor level, tumor stage, vascular invasion, or proliferation index. The expression level of miRNAs as suppressors, which can inhibit the expression of tumor promoting genes, is frequently down-regulated in breast cancer. Multiple lines of evidence also demonstrated the involvement of specific miRNAs as oncogenes in breast tumorigenesis. Some miRNAs have been considered to have potential clinical applications as a novel biomarker for breast cancer diagnosis and prognostic factor. Exploitation of the therapeutic potential of RNA interference will be an important task and achieved through the further understanding of the mechanisms of gene regulation by miRNAs.