Overwhelming experimental evidence accumulated over the past decade indicates that microRNAs (miRNAs) are key regulators of gene expression in animals and plants and play important roles in development, homeostasis, and disease. The miR-17-92 family of miRNA clusters is composed of 3 related, highly conserved, polycistronic miRNA genes that collectively encode for a total of 15 miRNAs. We discuss recent studies demonstrating that these miRNAs are essential for vertebrate development and homeostasis. We also show how their mutation or deregulation contributes to the pathogenesis of a variety of human diseases, including cancer and congenital developmental defects. Finally, we discuss the current evidence suggesting how the different miRNAs encoded by these 3 clusters can functionally cooperate to fine-tune signaling and developmental pathways.