MicroRNAs (miRNAs) are small non-coding RNA molecules with 21 to 24 nucleotides in length, which can regulate post-transcriptional gene expression by interacting with the 3' untranslated regions of the target mRNAs. MiRNAs are widely expressed in eukaryotic cells and involved in a variety of biological processes, such as in the development, differentiation, proliferation, and apop-tosis of cells. They also play essential roles in cell cycle regulation, migration, and tumor development. MicroRNA expression varies in different human tumors and is considered a powerful potential biological indicator in the development, diagnosis, treatment, and progno-sis of cancers. Breast cancer is one of the most common malignancies, and miRNA expression has been found to be differentially ex-pressed in various types of breast cancer. The expression and function of some miRNAs involved in breast cancer development, metasta-sis, and treatments are briefly summarized in this review.