MicroRNAs (miRNAs) are a recently discovered, extensive class of small noncoding RNAs that act as post-transcriptional regulators of gene expression. miRNAs have been implicated in most aspects of animal development and in pathological processes such as cancer. In neurons, miRNAs are expressed at all stages of development, and we start to learn about their involvement in neuronal differentiation, synaptogenesis, and plasticity. Moreover, miRNAs likely have a big impact on higher cognitive functions, and their relevance for the etiology of prominent neurological disorders is increasingly acknowledged. Future challenges include characterizing miRNA regulatory pathways on a system-wide basis, probing their relevance for in vivo brain function, and assessing their utility as molecular targets for therapeutic intervention.