Langevin equation: m·ẍ = -γẋ + √(2γkT)·η(t), where η is Gaussian white noise. In overdamped limit (large γ): diffusion coefficient D = kT/γ (Einstein relation). MSD = ⟨r²⟩ = 4Dt in 2D. This connects thermal fluctuations to dissipation (fluctuation-dissipation theorem).