MicroRNAs are widely expressed, approximately 22-nucleotide-long non-protein-coding RNAs. They are first transcribed as much longer primary transcripts, which then undergo a series of processing steps to yield the single-stranded, mature microRNAs. Like other classes of RNAs in a cell, the biogenesis of microRNAs is under the control of various internal and external cues. This review summaries our current knowledge of how the expression of mammalian microRNAs is regulated transcriptionally and post-transcriptionally.