This chapter introduces fusion energy and its challenges. Fusion offers constant, carbon-free energy that can be used day and night, unlike solar or wind, and is now moving from research to real-world delivery. Fusion systems are extremely complex and expensive. Fusion machines fuse hydrogen nuclei (deuterium and tritium) at extreme temperatures (exceeding 150 million degrees centigrade). Tritium is hard to produce and neutrons from fusion are damaging to reactor materials. Three types of fusion machine are described. Tokamaks use magnetic fields to confine plasma (the most common machine); stellarators which offer better stability of plasmas, but are harder to build; and, inertial confinement which uses lasers to compress fuel capsules. Three notable tokamaks are described: JET (UK), which operated 1983–2024 and achieved key breakthroughs; ITER (France), which is the world’s largest fusion experiment, still under construction; and STEP (UK), a prototype fusion power plant aiming for operation by 2040. As well as producing energy for electricity grids, fusion can directly power heavy industries and data centres. Its development has produced significant innovations in supportive technologies such as robotics, materials, and data science and modelling.