MicroRNAs, or miRNAs, are a recently discovered class of small regulatory RNAs that influence the stability and translational efficiency of target messenger RNAs (mRNAs). Alterations in miRNA expression are associated with an increasing number of biological processes, including breast cancer. The study of miRNAs 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 miRNAs, with a specific interest for breast cancer.