In eukaryotes, besides alternative splicing and promoter regulation of "classical" genes, there is also another level of genetic regulation based on non-coding RNAs (ncRNAs). The most famous group of ncRNAs is microRNAs, probably the biggest number of genome regulators. Here, we summarize the knowledge that has been accumulated about the microRNA field, focusing our attention on brief history, biogenesis, regulated mechanism, computational methods of miRNA finding and miRNA target sites, miRNAs and diseases, and miRNAs and network analysis.