Model reduction for linear Volterra integro-diffferential equations is studied. Generalized system Gramians are introduced and characterized as solutions to delay Lyapunov equations similarly arising for finite delay systems. The usual energy interpretation of the Gramians is provided and a reduced-order model of Volterra integro-differential type is obtained by truncation of a balanced system. An error bound for the H-2-norm is derived. It is further shown that particular choices for the Volterra kernel automatically yield approaches that have been studied in the literature. Additionally, the new approach allows us to also reduce time fractional systems. The method is numerically investigated by means of two spatially discretized partial differential equations.