There are several hundreds of known microRNAs, some of which are known to play important regulatory roles in animals by targeting the messages of protein-coding genes for translational repression. Misregulation of miRNA expression has an important role in development of many diseases. Although the first work on miRNA appeared in 1993, only in the last few years has the diversity of this class of small, regulatory RNAs been appreciated. One miRNA can regulate from a few to hundreds of genes, and since over 500 miRNA genes are present in higher eukaryotes, the regulatory gene network is important in various cellular functions. Several research groups have provided evidence that miRNAs regulate such cellular processes as early development, cell proliferation and death, apoptosis and fat metabolism, and cell differentiation.