We have measured the number of insoluble particles that attach to artificial interfaces under different flow conditions. In downward laminar flow, very few particles are attached even if there is a high concentration of particles near the interface and the number of particles attached is independent of the concentration of particles. In downward turbulent flow, the number of particles attached to an interface depends on the roughness of the surface, the direction of fluid flow, the size and density of particles, and the orientation of the interface. In horizontal flow, the dominant factor is sedimentation. An increase in fluid flow tends to reduce the number of particles attached to the interface, and thus the number of particles trapped. A crude criterion for the attachment of particles to a rough interface is proposed.