The impact of a drop on a rough solid surface is studied experimentally using a shadowgraph method with a rapid CCD camera system. Depending upon the physical parameters of impact a drop either spreads or splashes. For water drops colliding with aluminum surfaces, our experimental results for the onset of splashing on rough surfaces agree with those of Stow and Hadfield (1). The experimental results obtained here for various liquids and surfaces are correlated by an empirical formula relating the minimum "critical" Weber number for splashing to the ratio of the drop radius to the surface roughness coefficient. This formula was proposed by Wu (2) under the assumption that viscous forces can be neglected. By using various mixtures of water and glycerin, the critical Weber number for splashing is found to be indeed independent of viscosity. The formula is however not universal since two coefficients have to be adjusted for each liquid/surface combination. The real shape of the surface also introduces some discrepancy. Perturbations have been observed on the rim of the spreading film. These perturbations appear for a minimum impact velocity and their number depends on surface roughness.