All organisms go through cycles of growth and reproduction. Further enhancing the theme of genetic control in growth and development, this chapter covers the nuclear mechanisms (mitosis and meiosis) involved in cell reproduction and gamete formation. Many organisms have extremely complex life cycles or periods of growth during which genetic control is critical for the continued evolution of the species. Sexual reproduction is presented as a mechanism to increase genetic variability and to decrease the effects of deleterious mutations or combinations of genes in diploid organisms.