The problem of UV properties of primeval galaxies is briefly assessed from the theoretical point of view discussing its impact on the definition of the cosmological model.
Comments: 4 pages + 4 PS figures, LateX, Proceedings of the X Rencontres de Blois on "The Birth of Galaxies" (Blois, June 28 - July 4, 1998). Edited by B. Guiderdoni (Ed. Frontieres: Paris), in press. (see also http://www.merate.mi.astro.it/~eps/home.html)
