Nuclear energy has evolved from early uses of wood and fossil fuels through breakthroughs like Fermi's controlled nuclear reaction in 1942. It utilizes two reaction types - nuclear fission which controls reactions in reactor cores but produces radioactive waste, and future promise of cleaner and more powerful nuclear fusion. While nuclear energy is powerful, safely managing radioactive waste and ensuring non-proliferation remain challenges that if solved could realize its potential as a safe, clean energy source.