MicroRNA (miRNA) is a recently discovered new class of small RNA molecules that have a significant role in regulating gene and protein expression. These small RNAs (approximately 22 nt) bind to 3' untranslated regions (3'UTRs) and induce degradation or repression of translation of their mRNA targets. Hundreds of miRNAs have been identified in various organisms and have been shown to play a significant role in development and normal cell functioning. Recently, a few studies have suggested that miRNAs may be an important regulators of circadian rhythmicity, providing a new dimension (posttranscriptional) of our understanding of biological clocks. Here, we describe the mechanisms of miRNA regulation, and recent studies attempting to identify clock miRNAs and their function in the circadian system.