The many and very diverse, often contradictory, conditions to which a solar still must respond, at a very small investment, have been examined by the author. The solution arrived at is also described. This solution has also been adopted in the O.C.D.E. project of the Spanish government, for which the author is in charge of the scientific direction. These are large stills to feed a number of places in Spain, of which the first was started up at the beginning of 1966 at Las Marinas near Almeria, to supply a small agricultural village (900 A 2 still). A second large still (2,700 m 2) was to be constructed at the beginning of 1967 on the Isle of Tabarca off Alicante. After a description of the methods of carrying out the system and the materials chosen, an analysis of the investments necessary, and the sale cost of a cubic meter is given. Some considerations on the possibilities for the use of solar distillation are given.