The persistent yet highly variable stratospheric aerosol has impacts on radiation and stratospheric ozone, and thus climate. This Chapman Conference will address scientific questions that have grown out of observations over the recent period of quiescent to mild volcanism suited to study troposphere-to-stratosphere transport, background stratospheric chemistry, trends in stratospheric sulfate aerosol, and the sources and potential climate impact of non-sulfate stratospheric aerosol.