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Starting from small data, when does a nonlinear dispersive PDE have global solutions? A classical approach, just like for ODE, is to study resonances. But I will show that for PDE a new kind of resonances arises, that I call space resonances. This is the basis of a new method, that I will present; I will also show how it applies to a variety of equations of Mathematical Physics: non-linear Schrödinger, water waves, Euler-Maxwell... This is joint work with Nader Masmoudi and Jalal Shatah.