I will give an introduction to our theory for the formation of nuclear rings, which are remarkable gaseous structures commonly found at the centre of barred galaxies including our own Milky Way. The rotating external bar potential excites strong density waves near the inner Lindblad Resonance. The density waves remove angular momentum from the gas disc and transport the gas inwards. The accumulation of gas at the inner edge of the gap is the nuclear ring. The process has many similarities with the opening of gaps in protoplanetary disks driven by embedded planets & with the opening of gaps in planetary rings driven by satellites