Abstract MicroRNAs (miRNAs) are approximately 22-nucleotide-long non-coding RNAs that are important regulators of gene expression in eukaryotes. miRNAs are first transcribed as long primary transcripts, which then undergo a series of processing steps to produce the single-stranded mature miRNAs. This article reviews our current knowledge of the mechanism and regulation of mammalian miRNA expression and points out areas of research that may enhance our understanding of how the specificity and efficiency of miRNA production is controlled in vivo.