This lecture covers the principles and applications of free electron lasers (FELs), discussing different types of light sources, brilliance metrics, X-ray sources, 4th generation light sources, and science applications. It explains the resonance condition, microbunching, FEL modes, and the generic amplification process. The lecture also delves into electron beam requirements, undulators, motion in phase space, and self-amplification processes in FELs.