This lecture covers the influence of electromagnetic forces on electrons, microbunching, Pierce parameter, and gain length in X-ray free-electron lasers. It explains the concept of seeding and monochromatization, detailing the forces acting on electrons by EM radiation, SASE microbunching, and the intense energy spectra produced. The lecture also discusses the reasons behind the intensity of SASE microbunches and the techniques for seeding and monochromatizing SASE radiation.