MicroRNAs (miRNAs) are a class of regulatory RNAs that control gene expression at the posttranscriptional level. Binding to the target mRNA, miRNA suppresses its translation and/or induces its degradation. Although started relatively recently, studies of miRNAs have already shown that miRNAs are essential for switching the gene expression programs during embryo development and sustaining the normal cell function in the adult organism. Alterations of miRNA expression pattern and/or efficiency of mRNA binding seem to be as important in pathology as better known structural variations of proteins. The key role of miRNA has been demonstrated for the balance of cell proliferation, differentiation, and programmed death in several common diseases. The review considers the specifics of the miRNA-mediated regulation of gene expression and its role in normal and pathological development of muscle, immune, and nervous systems. Evidence is given for the involvement of miRNA in neurodegenerative and mental disorders.