RNA interference is a type of posttranscriptional gene silencing, when short RNA molecules suppress the function of RNAs and block gene expression. Double-stranded RNAs or short interfering RNAs injected into cells activate the RNA-induced silencing complex which degrades the target messenger RNA. The short RNAs produced inside the cell are called micro RNAs. These form a hairpin and then have the same function as double-stranded RNAs. RNA interference is an evolutionary important mechanism having a role in the protection against transposon and viral infection and regulate gene expression. While a number of studies demonstrate the in vivo applicability of RNAi, the first potential clinical trials are arising. So far it has been used to treat viral infections, inhibit macula degeneration, decrease the level of cholesterol in blood, treat cancer and neurodegenerative diseases. However, its application is hampered by ineffective bioinformatics algorithms unable to design effective short interfering RNAs, by low delivery efficiency and by the limited use to temporary antagonist gene silencing. The most important advantage of its application is the exceptional specificity resulting minimal side-effects. For this reason therapies based on RNA interference can be expected to spread in the near future.