Waves in the ultra‐low‐frequency (ULF) band (periods of 1 to 1000 s) are a permanent feature in the region called the magnetosphere, where the behavior of charged particles is influenced by the Earth's intrinsic magnetic field. These waves transport momentum and energy from the solar wind to the magnetosphere and farther down to the ionosphere. In addition, the waves interact with charged particles in the magnetosphere and modify their energy and direction of motion with important consequences for dynamic processes such as geomagnetic storms and the formation of auroral arcs.The physics of magnetospheric ULF waves is far from being completely understood primarily because many ULF waves have a wavelength comparable to the size of the magnetosphere, which is of the order of 10 Earth radii (RE). Just establishing the structure of such waves from observations is a daunting task. The source mechanisms for the waves are also often difficult to determine.