Weak protein–protein interactions (PPIs) are fundamental to many cellular processes, such as reversible cell–cell contact, rapid enzyme turnover and transient assembly and/or reassembly of large signaling complexes. However, structural and functional characterizations of weak PPIs have been technically challenging and lagged behind those for strong PPIs. Nuclear magnetic resonance (NMR) spectroscopy is a highly effective tool for unraveling the atomic details of weak PPIs.