The scattering of high energy electrons from nuclear targets has revealed, over decades of experiments, a rich and fascinatingly complex structure inside the nucleon. Its composition of quarks and gluons is governed by Quantum Chromodynamics (QCD), the theory of the strong interaction which is responsible for all hadronic matter in the universe. The bound state of quarks — nucleons and other hadrons — cannot, however, be directly calculated within QCD, nor are its properties entirely understood. What are the contributions to nucleon spin? How is the mass of a nucleon generated from the bare masses of its quarks, 1000 times lighter? What are the distributions of quarks and gluons inside a hadron and how do their dynamics determine its external properties? Recent advances in accelerator, detector and target technologies have enabled tomography of the nucleon at facilities such as Jefferson Lab, USA, opening the door into 3D study of the nucleon's internal dynamics.
