The exquisite fine tuning of biological electrical signalling is mediated by variations in the rates of opening and closing of different ion channels(1). In addition to open and closed conformations, ion channels can exist in an inactivated state, which prevents conduction in the presence of a prolonged activating stimulus(2). Human ether-a-go-go related gene (HERG) K+ channels undergo uniquely rapid and voltage dependent inactivation(3-5), which confers upon them a critical role in protecting against cardiac arrhythmias and sudden death(6). Previous structural studies have captured only the open state of the HERG channel(7,8). Here, we have exploited the K+ sensitivity of HERG inactivation to determine structures of both the conductive state and the elusive inactivated state of HERG. We show that hERG inactivation is facilitated by two competing networks of hydrogen bonds behind the selectivity filter that enable rapid and voltage dependent flipping of the valine carbonyls in the centre of the selectivity filter. Our data also explains how changes in extracellular K+ affects the distribution between conductive and inactivated states(9,10) and thereby explains why hypokalaemia reduces HERG channel activity thereby increasing the risk of cardiac arrhythmias(11).