The book reviews recent developments in the field of deep inelastic electron-proton reactions at high momentum transfers. These studies have been made feasible by the high-statistics "ep" collision data collected by the experiments ZEUS and H1 at the HERA accelerator at DESY. The standard model framework for describing deep inelastic scattering and scenarios beyond the standard model, as well as the experimental situation, is discussed, and the experimental results are collected and confronted with the theoretical predictions.