The Helfrich Hamiltonian H = ∫(κ/2)(2H)² dA describes elastic membranes like lipid bilayers. Thermal fluctuations excite bending modes; the spectrum ⟨|h_q|²⟩ = kT/(κq⁴) shows soft membranes undulate wildly at long wavelengths (Canham-Helfrich model).