Dynamic filtration experiments were conducted on cores with water based muds at different mud flow rates. It was observed that the thickness of the mud cake was a sensitive function of the mud rheology, the mud shear rate and the permeability of the rock. A model was developed to relate the process of cake buildup under dynamic conditions to the mud and rock properties. Excellent agreement between the model and the experiments was obtained. As predicted by the model (and observed experimentally) no cake was formed on rocks with low permeabilities when the overbalance pressure was reduced below some critical value. It has been generally accepted that no filter cakes form on rocks with very high permeability (lost circulation). It is shown in this paper that there is also a lower bound on the permeability below which no cake will form and that a minimum overbalance pressure must be maintained in the mud column to ensure mud cake formation. We also present equations that allow laboratory dynamic filtration results to be used directly to predict the critical permeability and the minimum overbalance pressure required for cake formation in the field