Flight in ground effect above a flat, smooth surface may give an animal considerable performance advantages, including a reduction in cost of transport of up to 15%, and a reduction in mechanical flight power of as much as 35%, compared with values for flight out of ground effect. Previous theories modelling the phenomenon have either been incomplete or marred by typographical errors. A complete lifting line theory of flight in ground effect with a fixed wing is developed, and instructions are given so that it may be applied to animals such as skimmers, pelicans and myotid bats which fly and forage close above water. Several predictions are made about likely flight behaviour in ground effect, and about the appropriate flight morphology for taking advantage of the potential performance improvements. The most important conclusion, differing from previous analyses, is that slow flight performance in ground effect is very poor, owing to the horizontal air velocity induced around the wing in the presence of the ground.