NMR is a powerful tool for quantifying the movements of protein side chains with atomic resolution. Observing the movements of the aromatic side chains is particularly informative: they are bulky but can however rotate up to 180 degrees to allow “ring-flips” if their local environment is favorable. Amyloid-like protein assemblies are very compact and their stability is explained by an almost perfect stacking of their side chains. By combining measurements of dynamics by solid-state NMR and an isotopic labeling strategy for aromatic nuclei, the Bordeaux and Grenoble sites of the NMR division of Infranalytics, in collaboration with Paul Schanda's team (IST Vienna) have characterized these famous “ring-flips” in amyloid fibers to demonstrate contrasting behaviors depending on their exposure to the fiber surface. This approach is applicable to a wide range of proteins, as illustrated by the studies of a 500 kDa enzyme, and crystal proteins, by the Grenoble team.