MicroRNAs (miRNAs) are an abundant class of approximately 22 nucleotide non-coding RNAs and play important regulatory roles in animal and plant development at the post-transcriptional level. Many miRNAs cloned from mouse bone marrow cells are differentially regulated in various hematopoietic lineages, suggesting that they might influence hematopoietic lineage differentiation. miR-181, a miRNA specifically expressed in B cells within mouse bone marrow, promotes B-cell differentiation when expressed in hematopoietic stem/progenitor cells. Some human miRNAs are linked to leukemias: the miR-15a/miR-16 locus is frequently deleted or down-regulated in patients with B-cell chronic lymphocytic leukemia and miR-142 is at a translocation site found in a case of aggressive B-cell leukemia. Collectively, these results indicate that miRNAs may be important regulators of mammalian hematopoiesis. Here, we provide background on the biogenesis and function of miRNAs and discuss how miRNA-mediated post-transcriptional regulation may influence the development and function of blood cells.