Gene regulation by microRNAs (miRNAs) plays important roles in development, physiology, and disease. miRNAs are an abundant class of noncoding RNAs that are generated through multistep biosynthetic pathways and typically repress gene expression through target destabilization and translational inhibition. Complex interactions between miRNAs and target mRNAs are associated with characteristic molecular mechanisms, including miRNA cotargeting, target-directed miRNA degradation, and crosstalk with various RNA-binding proteins. Consistent with the broad influence on cellular function, miRNA deregulation is commonly observed in various diseases, particularly cancer, with both tumor-suppressive and oncogenic roles. Mutations in the miRNA biosynthetic pathway and several miRNA genes have been linked to diverse types of cancer and a subset of genetic diseases, respectively. Additionally, super-enhancers play important roles in the regulation of cell type-specific and disease-associated miRNAs. This review summarizes the molecular features of miRNA biogenesis and target regulation along with the roles of miRNAs in disease biology, with recent examples expanding the pathophysiological roles of miRNAs.