Micro RNAs, or mi RNAs, are a recently discovered class of small regulatory RNAs that influence the stability and translational efficiency of target messenger RNAs (m RNAs). Alterations in mi RNA expression are associated with an increasing number of biological processes, including breast cancer. The study of mi RNAs is a rapidly developing field that could considerably change our vision of breast cancer biology. This book offers an insight into our current knowledge of human mi RNAs, with a specific interest for breast cancer.