(b) ULP, Strasbourg, France (c) CEA, SAp, Saclay, France

Clusters constitute one major source of information for cosmology. In the present paper we discuss their implication for the determination of the density parameter of Universe, $\Omega_0$. The classical dynamical argument is usually said to lead to low density universe. However, assuming that light is not biased in clusters lead to a wide range of values, between 0.2 and 0.65, differences been mainly due to the mass estimates. This method being possibly biased, new methods are requested for robust estimates. Two such global methods will be discussed here: the evolution of the abundance of clusters with redshift and the baryon fraction in clusters. The first method needs reliable estimates of the local temperature distribution function as well as at high redshift ($z \geq 0.3$).The second method can provide a direct estimate of $\Omega_0$ provide that we have a reliable estimate of the baryon content of clusters and a good estimate of $\Omega_b$ from primordial nucleosynthesis. Although it has become fashionable to claim that these tests lead to low values for the density parameter of universe, our most recent analyses lead to rather convergent values $\Omega_0 \sim 0.8$, and are nicely consistent with what can be inferred from CMB data.

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