This chapter aims to a topic of central importance in particle physics, the structure of hadrons as revealed by deep inelastic scattering experiments. At large momentum transfers there is very little elastic scattering: inelastic scattering, in which there is more than just the electron and proton in the final state, is much more probable. The simplest inelastic cross section to measure is the so-called ‘inclusive’ cross section, for which only the final electron is observed. The original discussion of the parton model took place in the infinite-momentum frame of the proton. While this has the merit that it eliminates the need for explicit statements about parton masses and so on, it also obscures the simple kinematic origin of the scaling. The particular frame is the one in which the photon and parton are collinear and the 3-momentum of the parton is exactly reversed by the collision. In this frame, the photon transfers no energy, only 3-momentum.