The main focus of this book is on experimental results from electron-positron and electron-proton colliders and related theoretical questions, particularly on hadron production at energies from 1 to 100 GeV and higher. The topics discussed include photo- and electroproduction of heavy flavours, the photon structure function, total cross section, jet production and resonance production. The future of the field is also discussed, notably experiments at linear photon-photon colliders.