The chapter discusses the effects of radiation on biological systems, focusing on both direct and indirect effects. Direct effects in polythene and dry bovine serum are influenced by oxygen and temperature but are independent of small amounts of water. Indirect effects, such as those in wet yeast cells, are more significant, with radiosensitivity being 2-50 times higher than in dry cells. The respiratory system of mouse-liver slices is affected by doses above 104 r, and the sensitivity is determined by cytochrome c, which is destroyed by irradiation. Studies on Drosophila show that X-rays can induce true reversions of mutations, and alkylating agents can produce point mutations in loci not observed under X-irradiation. The distribution of chemically and X-ray-induced mutations varies with the mutagen used. The chapter also covers the application of radiation mutagenesis in plant breeding, noting that only a small percentage of mutations in soybeans have economic value, while selection from irradiated cuttings in black currants has shown promising results. Additionally, the chapter touches on the stability of development in populations, highlighting the importance of heterozygosity and environmental heterogeneity in maintaining genetic diversity and adaptability.The chapter discusses the effects of radiation on biological systems, focusing on both direct and indirect effects. Direct effects in polythene and dry bovine serum are influenced by oxygen and temperature but are independent of small amounts of water. Indirect effects, such as those in wet yeast cells, are more significant, with radiosensitivity being 2-50 times higher than in dry cells. The respiratory system of mouse-liver slices is affected by doses above 104 r, and the sensitivity is determined by cytochrome c, which is destroyed by irradiation. Studies on Drosophila show that X-rays can induce true reversions of mutations, and alkylating agents can produce point mutations in loci not observed under X-irradiation. The distribution of chemically and X-ray-induced mutations varies with the mutagen used. The chapter also covers the application of radiation mutagenesis in plant breeding, noting that only a small percentage of mutations in soybeans have economic value, while selection from irradiated cuttings in black currants has shown promising results. Additionally, the chapter touches on the stability of development in populations, highlighting the importance of heterozygosity and environmental heterogeneity in maintaining genetic diversity and adaptability.