Techical information to be of assistance in the design of circuits using GIL: definition and description of GIL; general technical characteristics and their comparison with O/H lines and conventional cables; continuous and short-term current rating of directly buried GIL, sensitivity analysis to design parameters. Insulation and overvoltage aspects from a long term reliability point of view. Mechanical corrosion and installation aspects of directly buried GIL. Environment considerations. Economic aspects. Recent GIL projects and R&D programmes.