The complexity of multicellular organisms requires both an increase in genetic information and fine tuning in regulation of gene expression. One of the mechanisms responsible for these functions is RNA editing. RNA editing is a complex process affecting the mechanism of changes in transcriptome sequences. The best studied example of this process is A-to-I RNA editing. On the organism's level, RNA editing plays a key role during ontogenesis and in the defense against pathogens. Disorders in A-to-I RNA editing lead to serious abnormalities. The importance of RNA editing increases with an increase in the organism's complexity. Correct RNA editing is an indispensable factor of an organism's development and probably determines the lifespan of higher eukaryotes.