MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression by pairing with their target mRNAs, thereby inducing protein translation inhibition or/and mRNA degradation. There is now strong evidence that miRNAs play a crucial role in the regulation of hematopoiesis. Several groups have shown that miRNA expression change dynamically during hematopoietic differentiation and functional studies demonstrated that miRNAs control not only differentiation but also activity of hematopoietic cells by targeting transcription factors, growth factor receptors, and specific transcripts involved in the modulation of cellular responses to external stimuli. In this review, we will summarize the current knowledge of miRNA expression and function during hematopoiesis and discuss controversies and future directions. Am. J. Hematol., 2010. © 2010 Wiley-Liss, Inc.