Excitation spectrum of ferromagnetic Heisenberg chains

We present a review of the developments leading to our present understanding of the excitation spectrum of planar and uniaxial Heisenberg chains (xxz) as probed by dynamic form factors. Exact zero-temperature results, finite-chain calculations for s = 1/2, and molecular-dynamics results, realistic for s≫1 and T≳0, are discussed, and the role of envelope solitons clarified.