Fluorescent emitters like ions, dye molecules or semiconductor nanoparticles are widely used in opto-electronic devices, usually within densely-packed layers. Their luminescence properties can then be very different from when they are isolated, because of short-range interparticle interactions such as Förster resonant energy transfer (FRET). Understanding these interactions is crucial to mitigate FRET-related losses and could also lead to new energy transfer strategies. Exciton migration by FRET hopping between consecutive neighbor fluorophores has been evidenced in various systems but was generally limited to distances of tens of nanometers and involved only a few emitters. Here we image self-assembled linear chains of CdSe nanoplatelets (colloidal quantum wells) and demonstrate exciton migration over 500-nm distances, corresponding to FRET hopping over 90 platelets. By comparing a diffusion-equation model to our experimental data, we measure a (1.5 ps)-1 FRET rate, much faster than all decay mechanisms, so that strong FRET-mediated collective photophysical effects can be expected. FRET is a non-radiative energy transfer mechanism by electromagnetic dipole-dipole coupling 1. FRET energy migration plays a key role in photosynthesis by funneling photogenerated excitons from the light harvesting centers to the reaction center 2. Systems of dyes attached to DNA scaffolds have been engineered in order to create FRET-based bio-inspired 30-nm photonic wires 3 and logic gates 4. In opto-electronics, FRET must be understood as it may be detrimental by funneling excitonic energy to quenching sites, but it can also favor efficient charge collection in photovoltaic cells and lead to new FRET-enabled excitonic devices 5. With this in mind, exciton migration has been explored in various systems. In molecular or polymer systems, migration by incoherent FRET hopping was generally demonstrated over a few tens of nanometers 6-8 (while distances of hundreds of nanometers or microns could be reached by other mechanisms such as Dexter hopping and coherent exciton motion 6,9-10 and by a combination of coherent and incoherent exciton motion in the case of J-and H-aggregates 11, 35, 36). Within quantum dot films, various methods were developed for the delicate characterization of FRET exciton migration 12 and yielded diffusion distances of 10-30