Deuterium, lithium and the density of the Universe

The main outcome of the primordial nucleosynthesis is the ability to account for the abundances of D, ^sup 3^He, ^sup 4^He and ^sup 7^Li with the proper choice of the nuclear density parameter ω^sub B^. The relative advantages/disadvantages of D and ^sup 7^Li as the proper `baryometer' are discussed. In favour of D, the main arguments are the relative simplicity of the formation/destruction schema, but this is challenged by the large uncertainties on the choice of its actual `primordial' abundance and on the galactic evolution scheme. In favour of ^sup 7^Li there are the confirmation of the so called `Spite plateau' and the observation of ^sup 6^Li at the surface of at least one (may be two) Population II stars, but the paucity of such stars such as the possibility of scenarios in which the ^sup 7^Li abundance could be affected even in these stars cannot be overlooked.[PUBLICATION ABSTRACT]