The brain is of bewildering complexity and numerous genes and signaling molecules have been described that affect the architecture and functioning of specific neuronal circuits. Recent evidence from genome analysis revealed the existence of a large group of novel RNA molecules with unexpected properties. One such group is called microRNAs, which are small 21-23 nucleotides RNA molecules that are transcribed by the genome. However, they are not translated into proteins but rather control translation of coding mRNA. Particularly in the brain, numerous different microRNAs are expressed in a cell type specific fashion both during development and in adulthood. Aberrant microRNA expression has been implicated in several human diseases including CNS diseases. The aim of this review is to emphasize their role in the development of the brain and their function. In addition, we highlight recent findings on the evolution of mammalian microRNAs and their effect on steroid signaling in the brain.