Nuclear genome size differs thousands-fold in animals and does not correlate with their com- plexity. Such pattern results from domination of non-coding, most often repetitive sequences in genomes. Most common types of repetitive sequences and their origin are mentioned. Then, some hypotheses on the origin of so great variability in the amount of non-coding DNA are presented. According to the present state of knowledge, a cause-and-effect chain exists, from the amount of DNA, through nucleus volume, to cell size. Because cell size affects strongly the metabolic rate and developmental rate, the amount of DNA and mechanisms changing this amount must be also under strong selection pressure. Thus, the variability of DNA amount reflects the variability of mutation-selection balance among organisms.