The drilling of horizontal wells has led to new demands on drilling fluid design. Less damaging drill-in fluids are formulated after extensive geological testing of the productive formation. This paper describes the main tests undertaken and some of the fluids commercially available. Geological analysis of the formation is followed by the testing of a proposed fluid, including extended exposures of the formation and fluid in conditions imitating those found downhole. Potential production impact is assessed by a return permeability test. With a clear understanding of what causes formation damage, drill-in fluid chemistry and properties can be engineered to prevent damage. Glycol, formate and sized calcium carbonate particle technologies are described.