Micro (mi)RNAs are small, regulatory RNA molecules that are integral components of the genetic program in the majority of cells. They are thought to regulate up to one third of all human genes and have been linked to critical processes in disease pathogenesis. The diverse role of miRNAs in disease pathogenesis suggests that the modulation of miRNA function by utilizing techniques such as the use of antagomirs, locked nucleic acids, or miRNA sponges may produce novel therapeutic approaches. In this review, the current understanding of post-transcriptional gene regulation by miRNAs is discussed and insights into the function of miRNAs in tumorigenesis, immune responses, muscle function, organogenesis, and cell-lineage decisions are reviewed.