Breast cancer is the leading cause of cancer death in women worldwide. Gene expression studies have been used over the last decades to defi ne the signature of different breast cancer subtypes and to predict outcome and response to therapies. Recently, microRNAs (miRNAs) have been linked to several human diseases, including cancer. An aberrant miRNA expression in breast cancer was fi rst reported in 2005. Now, an increasing body of experimental evidences supports the role of these small molecules in the tumorigenic process and their potential use as cancer specifi c biomarkers. Indeed, miRNAs are detectable as circulating molecules in the blood. In this chapter, we summarize our knowledge about the involvement of miRNAs in breast cancer and their potential as diagnostic, prognostic and therapeutic tools.