MicroRNAs (miRNAs) constitute a group of single-stranded, noncoding RNAs that regulate the expression of structural genes on post-transcriptional level. miRNAs are transcribed as several hundred- nucleotide pri-miRNAs. The mature, active form is only 21–23 nucleotides long and it binds target mRNAs containing antisense sequences. miRNA can either catalyze cleavage of mRNA that are perfectly base- paired to its sequence, or inhibit the translation of mRNAs that form an imperfect complex with the miRNA. Hundreds of miRNAs are described to date, which populate the genomes of plants and animals. They are necessary for crucial cellular and developmental processes. Some of them directly regulate the development; some affect programmed cell death (PCD), some at last are essential for signal transduction. miRNA expres- sion profiles classify human cancers better than mRNA profiles what may be of big diagnostic and therapeu- tic value. This study presents the current understanding of miRNA biogenesis, regulatory mechanisms and the role that miRNAs exert in embryogenesis, hematopoiesis and oncogenesis.