MicroRNAs are small non-coding RNA which control gene expression by degradation of protein-coding transcripts or through translational repression. They were discovered in 1993 so their roles are still matter of research. The fact that the miRNAs are often part of a complex regulation network and impact the expression of a large number of genes, makes research a bit difficult. As well as the fact that a single gene can regulate many different miRNA molecules. Therefore, miRNAs have important role in the development of malignancies where, depending on changes in their regulation, they can act as oncogenes or tumor-suppressor and participate in the development of all six major characteristics of malignant diseases. Both genetic and epigenetic mutations can cause changes in the level of expression of microRNAs because microRNAs are not only target, but also have a regulatory role of other genes by influencing the epigenetic effectors. The tissue-specific expression of many microRNAs and their stability in extracellular fluids makes them promising biomarkers, while the fact that the impact on the number of changes due to malignant disease makes them attractive targets of new drugs. Therefore, microRNAs are one of the possibilities for a better understanding of malignant diseases.