This paper presents an overview of the work carried out to date on the dynamic and static filtration characteristics of drilling muds. Factors controlling fluid loss to the formation that have been investigated include annular velocity, fluid temperature, hole angle, shear rate and the effect of the filter media. The effect of pressure on static fluid loss and filter cake permeability is also reported. Fluid loss during sequential filtration has also been evaluated. The fluids used comprised a calcium chloride brine with fluid loss additive and two water based muds. The filter media consisted of synthetic core material and a natural sandstone. Conventional photographic techniques have been used to idenify filter cake response to backflushing.