MicroRNAs (miRNAs) are a class of RNA which controls gene expression at the posttranscription level. Binding with the target RNA, a miRNA can supress translation and/or induce degradation of mRNA. The studies of miRNAs have already shown that miRNA were essential to switch the programs of gene expression during embryo development as well as to control cell functioning of the adult organism. Alteration of the miRNA expression profile may appear not less important in development of pathology than better known structural variations of the proteins. The role of miRNA has been confirmed for a range of common diseases connected to impaired balance of cell proliferation, differentiation and programmed death. This review discusses specific features of miRNA-mediated regulation of gene expression and its role in normal and pathological development of muscle, immune, and nervous systems. The evidence of miRNA involvent in neurodegenerative disorders and mental disorders is demonstrated.