Out-of-time-order correlators (OTOCs) measure how quantum information spreads through a many-body system. The OTOC C(x,t) = -⟨[W(x,t), V(0)]²⟩ decays after a scrambling time, with a wavefront propagating at the butterfly velocity v_B. This is the quantum analog of the classical Lyapunov exponent — it saturates the chaos bound λ_L ≤ 2πk_BT/ℏ (Maldacena-Shenker-Stanford).