In this work, we investigate the Perfectly
Matched Layers (PML)
introduced by Bérenger  for designing
efficient numerical absorbing
layers in electromagnetism.
We make a mathematical analysis of this model, first via a modal
analysis with standard Fourier techniques, then via energy
techniques. We obtain uniform in time stability results (that make
precise some results known in the literature) and state some energy
decay results that illustrate the absorbing properties of the
model. This last technique allows us to prove the stability of the
Yee's scheme for discretizing PML's.