RNA interference (RNAi) represents a mechanism invented by nature to protect the genome. In the past few years the field has emerged at a surprisingly high pace. The underlying molecular mechanism of gene silencing provides us with short interfering RNAs (siRNAs) which allows to target any gene with high specificity and efficiency. siRNAs can now be obtained in various ways allowing for numerous in vitro and in vivo applications. Successful knock-downs of disease-related genes indicate that siRNAs open the door for novel therapeutic procedures.