MicroRNAs are short non-coding RNAs involved in post-trascriptional regulation of gene expression. They can be found in various organisms including animals, plants and viruses and they play a key role in diverse biological processes, such as embryogenesis, differentiation and proliferation of cells, production of cytokines or apoptosis. Since the initial observation, more than 700 microRNAs have been identified in mammalian cells and up to one third of all protein-encoding genes is estimated to be regulated by these small molecules. In order to biological functions of these mRNA targets, it is apparent today, that altered microRNA expression can contribute not only to carcinogenesis but also to serious failure of the immune system. The aim of this review is to summarize the latest findings about the biogenesis and function of microRNAs and to refer on their role in the regulation of ontogenesis of immune system, haematopoiesis, immune responses and viral infections. Possible contribution of microRNAs to the therapy of viral infections is discussed.