abstract: We consider geometric flows, where a metric evolves by \partialt g = -2 S for some symetric two-tensor S. For these flows we define forwards and backwards reduced volume quantities in the spirit of Perelman's backwards reduced volume for the Ricci flow. We show that if S satisfies a Harnack type inequality along the flow these volume quantites are monotone. This result implies new monotonicity formulas for flows like Bernhard List's extended Ricci flow system or the mean curvature flow in certain Lorentzian manifolds.