A significant amount of lightning impulse breakdown and withstand testing has been accomplished in mineral oil to understand this important electrical characteristic for transformer insulation system design. To be considered as viable insulating fluids for high voltage equipment, alternative dielectric liquids should have similar lightning impulse characteristics as compared to mineral oil to provide the clearances necessary for common dielectric design. This paper reviews the testing for establishing the lightning impulse breakdown characteristics of natural ester fluids relative to mineral oil test results. The key variables of the testing included: various oil gap and solid insulation creep distances, electrode configurations, electrical stress characteristics, and solid insulation surfaces. The fluid gaps ranged from 3 mm to 55 mm. The electrode configurations included quasi-uniform in oil gaps, with some attached to pressboard, and nonhomogeneous contacts in oil gaps combined with a phenolic interface. The range in gap distance was selected to reflect the range commonly used in liquid insulated transformer core/coil designs. The electrical stresses used included 1.2 Ã 50 Â¿s lightning impulses of both positive and negative polarity. The solid insulation materials consisted of high density pressboard, Kraft paper and a high density phenolic composite. It is concluded that the impulse breakdown voltage of the natural ester fluid is similar to mineral oil for the oil gaps and electrode configurations tested.