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A founding principle of structural biology is that, once researchers can directly observe macromolecules in enough detail, it should be possible to understand how their 3D structures confer their biological functions. Indeed, many scientific advances have relied on directly observing the world around us in as much detail as possible, and efforts are increasingly being dedicated to visualizing the atomic structures of biological components that have a key role in human disease. Nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography and cryo-electron microscopy (cryo-EM) are the three main structural-biology techniques in use. Of the three, cryo-EM has emerged as the current ‘go to’ method for determining the structures of large and dynamic complexes that have proved difficult to obtain by the other approaches.