The notion that protein-coding genes are the only biologically significant moieties in the genome is revolutionarized by the discovery of non-coding RNAs. Among them is a group of small non-coding RNAs termed microRNAs (miRNAs) that has recently been brought to the limelight. Their presence in the genome was only realized after the identification of their founders lin-4 and let-7 in C. elegans through genetic screens. To date, several hundreds of microRNAs have been identified. They are single-stranded RNA molecules of ∼21-22 nucleotides long, excised from precursors of ∼70 nucleotides predicted in silico to from imperfect stem-loop structures. Many of these miRNAs are evolutionarily conserved, implicating important biological functions. Emerging data points to an important role of these tiny RNA molecules in regulating gene expression of target RNAs by at least two post-transcriptional mechanisms: binding to imperfect complementary sequence of the 3 untranslated region (3UTR) and repressing translation; and endonucleolytic cleavage by a double-stranded RNA-mediated interference (RNAi) mechanism through base-pairing with perfect or near-perfect complementary sequence. Studies in several miRNAs in plants and animals illustrate that these small riboregulators can play critical roles in developmental timing and spatial control of cell fate decisions, cell proliferation, apoptosis, cell differentiation and cell metabolism. Developmental defects and tumorigenesis can be a consequence of abnormal expression of miRNAs. This review discusses the biological functions, mechanisms of action, biogenesis and regulation of miRNAs, as well as identification of their target genes, highlighting the potential diversity of this novel class of gene regulators.