This chapter reviews research on the effects of drilling fluid on reef-building, or hermatypic, corals. Experiments have shown that the burial of corals in drilling fluid, or mud, caused mortality and that certain drilling fluids applied as slurries could not be removed by corals. Under field conditions, however, slurries were removed with the assistance of natural currents, but appeared to cause lowered growth rates in treated specimens. Certain species showed behaviorial stress symptoms after 96-h exposure to 0.100 ml liter-1 of drilling fluid (0.100 ml of fluid in 1 liter of seawater) and exposure to 1.000 ml liter-1 caused mortality in 65 h for three of seven species tested. A chronic 6-week exposure to 0.100 ml liter-1 of drilling fluid caused an 84% decrease in calcification, a 40% decrease in respiration, reductions in gross photosynthesis (26%), nitrate uptake (28%), ammonium uptake (49%), and feeding, as well as some death. Other studies showed that average linear skeletal growth also decreased. A field assessment of a reef, several years after drilling , indicated a 70-90% reduction in foliose, branching, and platelike corals within a 115 m x 85 m ellipse around the drilling site. Detrimental effects on corals, as extrapolated from the limited information on effects, seem probable within a minimum distance of 100m from the source.