Single-molecule FRET (smFRET) has become an established tool to study biomolecular structure and dynamics in vitro and in live cells. We performed a worldwide blind study involving 19 labs to assess the uncertainty of FRET experiments for proteins with respect to the measured FRET efficiency histograms, determination of distances, and the detection and quantification of structural dynamics. Using two protein systems that undergo distinct conformational changes, we obtained an uncertainty of the FRET efficiency of less than ± 0.06, corresponding to an interdye distance precision of ≤ 0.2 nm and accuracy of ≤ 0.5 nm. We further discuss the limits for detecting distance fluctuations with sensitivity down to ≲ 10% of the Förster distance and provide guidelines on how to detect potential dye perturbations. The ability of smFRET experiments to simultaneously measure distances and avoid averaging of conformational dynamics slower than the fluorescence lifetime is unique for dynamic structural biology. ### Competing Interest Statement Tim Craggs and Achilles Kapanidis, two of the authors are founders of different companies selling single-molecule fluorescence microscopes (Exciting Instruments, Oxford Nanoimager).