fax 01-972-952-9435. AbstractTo be able to keep a well under control during drilling it is important to have the weight material suspended during the whole operation. Especially in oil based, or synthetic based drilling fluids, controlling sag is difficult. A large number of wells have experienced lost circulation or well control problems with the associated high costs relating to sag. There are limited field proven methods available to predict the potential for sag for a particular drilling fluid. Therefore, barite sag continues to be one of the most difficult tasks to handle for a drilling fluid engineer. It is well known that standard viscosity readings cannot be used to predict the sag potential of a drilling fluid.The current paper discusses in detail the mechanisms of sag in oil based drilling fluid and how to treat sag. It is shown that at ultra-low shear rates the oil based drilling fluid is shear thickening as opposed to the shear thinning effect observed at higher shear rates. It is described how this shear thickening behaviour leads to sag if the drilling fluid composition is incomplete. It is described how organophilic clay should be used to minimize sag and why different types of clay have different effect onto hindering sag although none of these differences have to be visible on standard viscometer readings. Furthermore, it is suggested that "modifiers" generally have little effect on preventing sag. Different methods to monitor or predict the sag potential of drilling fluids are also outlined. Dilution of the drilling fluid normally results in increased sag. Good practice is that the dilution with new drilling fluid should be minimized to control the sag properties of the oil based drilling fluid.