The genomes of all living organisms are extremely stable. DNA in the cell is replicated during cell division and passes all the genetic information to the next generation. It is essential for all living organisms to ensure proper functioning and propagation of their genetic information. Due to constant exposure of the genome to various endogenous and exogenous agents, however, the DNA becomes damaged leading to a large variety of DNA lesions. The endogenously generated damage of DNA is known as spontaneous DNA damage, which is produced by reactive metabolites and defects in normal processes of DNA replication or recombination. The exogenous DNA-damaging agents include chemical mutagens, for example base analogs, alkylating agents and aromatic compounds, and physical mutagens: UV light, ionizing radiation and high temperature. The DNA lesions produced by these damaging agents could result in a base modification, such as alkylation and oxidation, base deletion, cyclobutane dimers, 64 photo-products, strand breaks, intra- and interstrand cross-links. Those damages of DNA have genotoxic or cytotoxic impact to the cell. Although most mutations are either neutral or harmful, some mutations have a positive effect on an organism. In this case, the mutation may enable the mutant organism to withstand particular environmental stresses better than wild-type organisms, or to reproduce more quickly. Mutagenesis is used to induce mutations at high frequency that include ionizing radiation and chemical mutagens for basic plant research and plant breeding.