The size of the genome of angiosperms, although determined only for 1.4% of described species, is very diverse. Comparative genomic studies have shown that differences in genome size result mainly from differences in the amount of repeated sequences, whereas the number of genes is similar. The role of non-coding repetitive sequences is controversial. These hypotheses indicating their parasitic nature, as well as data about their adaptive significance. Unexplained are the mechanisms leading to increasing the size of the genome, and even more so for its reduction. Molecular studies attribute an important role in the process of moving sequences especially retrotranspozonom. The size of the genome is a characteristic of a given species, but may be subject to change. Knowledge of genome size is not only important from the point of view of basic biological research, but may also have practical significance.