The complex chemistry of clays and an understanding of their structures provide the opportunity for manipulating their rheological properties. The application of bentonite in the oilfield is still widespread and when used smartly can be a very effective tool. Clay is economically advantageous and environmentally friendly. Clay, when applied together with a metal oxide extender produces a truly unique fluid capable of fulfilling the most demanding hole cleaning tasks. Simple MMO systems occasionally require additional fluid loss control if high permeability zones are exposed.