The focusing nonlinear Schrödinger equation iψₜ + ½ψₓₓ + |ψ|²ψ = 0 exhibits modulational instability: a uniform plane wave ψ₀ = A·eⁱᵏˣ is unstable to small perturbations at wavenumbers |q| < 2|A|. Growth rate Ω(q) = |q|√(2A²−q²/2). This Benjamin–Feir instability underlies rogue wave formation and nonlinear wave turbulence.