The flocculation of colloidal particles by adsorbing polymers is a phenomenon of considerable scientific and industrial importance. This article describes recent developments in the field with particular reference to flocculation of negatively charged particles by high-molecular-weight cationic polyelectrolytes by polymer bridging or charge neutralization. Experimental and theoretical studies of interactions between adsorbed polymer layers give insight into the favourable conditions for bridging flocculation. Neutron scattering and computer simulation give insight into the relationship between floc structure and the mechanism of flocculation.