Foaming of water based muds can be very costly and disruptive to the whole drilling process. Excessive foaming causes pump problems and can lead to well control issues as mud pit volumes cannot be monitored properly. Foaming is difficult to break down because it is stabilised by the surface elasticity and surface viscosity of its film. Alcohol defoamers have an affinity for the air-liquid surface where they destabilise the foam. Drilling muds containing dissolved salts and some shale inhibition products are more prone to foaming and defoamers should be programmed on these wells. Defoamer and Antifoam are interchangeable terms when applied to drilling muds.