  We develop a semi-classical approximation to electron spin resonance in
quantum spin systems, based on the rotor or non-linear sigma model. The
classical time evolution is studied using molec- ular dynamics while random
initial conditions are sampled using classical Monte Carlo methods. Although
the approximation may be especially powerful in two dimensions, we apply it
here to one- dimensional systems of large spin at intermediate temperatures, in
the presence of staggered and uniform magnetic fields. We first test the
validity of the semi-classical approximation by comparing the magnetization to
quantum Monte Carlo results on S = 2 chains. Then we calculate the ESR
spectrum, finding broad coexisting paramagnetic and spin wave resonances.
