Prospects for ultra-high voltage (1000 kV and above) a.c. power transmission are discussed, and the problems involved in developing and constructing this u.h.v. transmission are reviewed. Research above 1000 kV is concerned with minimizing the impact of u.h.v. lines on the immediate environment, particularly radio and audible noise emission due to wet-weather corona effects, and electrostatic induction phenomena near the lines. The paper reviews the design constraints now envisaged to avoid these problems, as well as insulation requirements that may be necessary. No specific technical limitations through 1500 kV have yet been encountered, although the formation of power corridors with limited public access may be necessary above 1500 kV.