Nonlinear dispersive waves interact via resonant triads (3-wave) or quartets (4-wave). Energy cascades through scales following the Zakharov kinetic equation. Power-law spectrum E(k) ~ k-α analogous to Kolmogorov.
Resonance: ω₁ + ω₂ = ω₃, k₁ + k₂ = k₃. Zakharov (1965): exact power-law stationary solutions.