COPII mediates Endoplasmic Reticulum to Golgi trafficking of thousands of cargoes. Five essential proteins assemble into a two-layer architecture, with the inner layer thought to regulate coat assembly and cargo recruitment, and the outer coat forming cages assumed to scaffold membrane curvature. We have visualised the complete, membrane-assembled COPII coat using cryo-electron tomography and subtomogram averaging to reveal the role of cargo binding and the effect of the biophysical properties of the membrane on COPII-coated membrane carrier morphology.