Since the discovery of microRNAs (miRNAs) in Caenorhabditis elegans, mounting evidence illustrates the important regulatory roles for miRNAs in various developmental, differentiation, cell proliferation, and apoptosis pathways of diverse organisms. We are just beginning to elucidate novel aspects of RNA mediated gene regulation and to understand how heavily various molecular pathways rely on miRNAs for their normal function. miRNAs are small non-protein-coding transcripts that regulate gene expression post-transcriptionally by targeting messenger RNAs (mRNAs). While individual miRNAs have been specifically linked to critical developmental pathways, the deregulated expression of many miRNAs also has been shown to have functional significance for multiple human diseases, such as cancer. We continue to discover novel functional roles for miRNAs at a rapid pace. Here, we summarize some of the key recent findings on miRNAs, their mode of action, and their roles in both normal development and in human pathology. A better understanding of how miRNAs operate during the normal life of a cell as well as in the pathogenesis of disease when deregulated will provide new avenues for diagnostic, prognostic, and therapeutic applications.