  Spin excitation spectrum of CMR manganites are studied from both theoretical
and experimental sides. Scaling relations for the spin wave lifetime in the
electronic model with double-exchange interaction is observed, which is
consistent with results for neutron inelastic scattering experiments for
(La,Y)$_{0.8}$Sr$_{0.2}$MnO$_3$. Roles of other interactions and degrees of
freedom such as polaron effects are also discussed.
