  We provide the rigorous derivation of the wave kinetic equation from the
cubic nonlinear Schr\"odinger (NLS) equation at the kinetic timescale, under a
particular scaling law that describes the limiting process. This solves a main
conjecture in the theory of wave turbulence, i.e. the kinetic theory of
nonlinear wave systems. Our result is the wave analog of Lanford's theorem on
the derivation of the Boltzmann kinetic equation from particle systems, where
in both cases one takes the thermodynamic limit as the size of the system
diverges to infinity, and as the interaction strength of waves or radius of
particles vanishes to $0$, according to a particular scaling law
(Boltzmann-Grad in the particle case).
  More precisely, in dimensions $d\geq 3$, we consider the (NLS) equation in a
large box of size $L$ with a weak nonlinearity of strength $\alpha$. In the
limit $L\to\infty$ and $\alpha\to 0$, under the scaling law $\alpha\sim
L^{-1}$, we show that the long-time behavior of (NLS) is statistically
described by the wave kinetic equation, with well justified approximation, up
to times that are $O(1)$ (i.e independent of $L$ and $\alpha$) multiples of the
kinetic timescale $T_{\text{kin}}\sim \alpha^{-2}$. This is the first result of
its kind for any nonlinear dispersive system.
