MicroRNAs (microRNAs) are abundant, approximately 21-25 nucleotide (nt) non-coding RNAs that mediate sequence-specific, post-transcriptional repression of mRNA targets. Emerging evidence suggests that several microRNAs target transcripts that encode proteins directly or indirectly involved in cell cycle progression and cellular proliferation. Moreover, alteration of microRNA levels can contribute to pathological conditions, including tumorigenesis, that are associated with loss of cell cycle control. In this review we highlight recent data linking microRNAs to mammalian cell cycle regulation. We describe how specific miRNAs function within pathways that control cell cycle checkpoints. We discuss emerging evidence that support the idea that some microRNA activity may be cell cycle dependent, and we outline how the coordinate regulation of microRNA targets may influence cell cycle progression.