microRNAs (miRNAs) are small non-coding RNAs conserved in metazoans. Depletion of miRNAs results in embryonic lethality, suggesting they are essential for embryogenesis. Similarly, pathways induced by growth factors of the transforming growth factor β (TGF-β) superfamily control cell growth, differentiation, and development. Recently Smad proteins, the signal transducers of the TGF-β pathway, were found to regulate miRNA expression, which, in turn, affects expression of numerous proteins. Smads modulate miRNA expression through both transcriptional and post-transcriptional mechanisms illustrating the complexity of gene regulation by TGF-β. In this chapter we summarize the current knowledge of mechanisms underlying Smad-mediated regulation of miRNA biogenesis.