Abstract. In his 1900 Festschrift article, dedicated to the 25th anniversary of Lorentz' dissertation, Henri Poincaré derived an equation expressing conservation of momentum in electromagnetic systems. I describe his proof and then proceed to an example illustrating it. It turns out that Newton's third law as a law solely between interacting charges does not always hold true. Rather we must also take into account the momentum stored in the electromagnetic fields in order to conserve momentum. This generalization of momentum conservation to include the momentum of the electromagnetic field led Poincaé in his paper to mass energy equivalence for the energy content of his "fluide fictif", making it probably the first assertion about E=mc² as we have come to understand it.

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