Grouting is a procedure in which grout is injected into different kinds of spaces. This procedure has had broad applications in modern civil engineering, especially in karst terrains, for nearly 200 years. Ingredients for the preparation of mortars and grouting suspensions include cement, clay and fillers, bentonite, asphalt, additives for stability and water. The building of dams in karst areas is always accompanied by the construction of large grout curtains. During their construction, thousands of tons of materials are injected into the karst underground. Some ingredients and chemicals used in the preparation of mortars and grouting suspensions may be toxic, neurotoxic or carcinogenic, and may be irritants or corrosives. Their use is dangerous to both humans and the environment. The toxic components can pollute karst aquifers and cause long-lasting hazardous consequences on karst underground species. Both physically and chemically, this material voraciously and quickly destroys underground habitats and kills an enormous number of rare, endangered and endemic species. This is an extremely expensive procedure although, in many cases, it is not very successful from the engineering perspective. When the construction of a grout curtain is completed, the hydrostatic pressure upgradients of the curtain are increased to higher levels than ever. These high hydraulic gradients accelerate the dissolutional expansion of fractures and bedding planes beneath the dam by several orders of magnitudes. As a result, intolerable leakage of karst reservoirs can occur over the lifetime of a dam site. This paper provides several examples of varying negative impacts of grouting on the hydrological and ecological regimes of the karst environment in the Dinaric karst. Copyright © 2009 John Wiley & Sons, Ltd.