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Publisher Summary This chapter provides a simplified and heuristic approach to the free electron laser (FEL) that frames it within the context of coherent sources. The FEL is a nonconventional laser device. Powerful coherent light can be generated either by the stimulated emission of an inverted atomic or molecular medium or by free electrons moving in a waveguide or an undulator magnet. The chapter emphasizes the various effects of the dynamic behavior of the Compton FEL interaction, including the spontaneous emission, gain effects, transverse, longitudinal mode dynamics, and saturation mechanisms. The goal of the chapter is to provide simple recipes to calculate the gain when inhomogeneous broadening effects are active, or the effect of optical mode focussing induced by the interaction, and to describe the interplay between saturation and lethargy. The FEL is becoming a source of coherent radiation with the unique feature of its wide tunability.