In the microfiltration of colloids it has become apparent that there is a critical Flux below which there map be no fouling of the membrane by the colloids. The literature supporting this conclusion is reviewed and experimental results presented which show that there is no fouling of the membrane in laboratory and full scale systems operated below the critical flux in the region which is termed sub-critical. Operation above the critical flux however causes fouling which reduces flux to the critical value over time. Increasing transmembrane pressure only increases flux for a transient period with the stable flux eventually falling to the critical flux. Reduction of transmembrane pressure causes the flux to fall below the critical region if fouling of the membrane has already occurred. It may be possible once flux is in the sub-critical region for spontaneous cleaning to occur. The value of the critical flux is known to be a function of particle size, hydrodynamics and membrane-colloid interactions.