Many landfills in Europe have to be covered in the near future and the use of steel slags as construction material seems to be sensible. This work compiles the demands on landfill cover materials and points out problems that can occur when using steel slag in a landfill cover. There are only few legal requirements for materials used in landfill cover constructions. The most important demand on a non hazardous landfill cover is a permeability of less than 50 lm Y2 yrY1. Four electric arc furnace (EAF) slags and one ladle slag were investigated on their chemical and physical properties. One EAF slag turned out to have a high molybdenum mobility so that its use is problematic. The field capacities of some EAF slags were between 17 and 38% which is in accordance with the requirements. Ladle slag hardens fast when mixed with water which can cause difficulties in practical application.