  The baryon fraction in galaxy clusters is one of the most direct way to
constrain $\omo$. The baryonic fraction is estimated to be in the range fb =
(0.15 - 0 .20) h^{-3/2} which is several times higher than expected from the
observed light element abundances in $\omo=1$ universe, leading to the
conclusion that we live in low density universe. In this work I will first
present the various steps which lead to these results and then foccus on the
distribution of the baryon fraction inside clusters. I will show that while the
baryon fraction in clusters follows a scaling law, its theoretical expected
shape does not agree with what is inferred from observations.
  I will show that when various systematics entering into the determination of
clusters gas fraction are taken into account the observed baryon fraction
profile matches better the predictions, provided that the average baryon
fraction at the virial radius is lower than previously found fb~10%.
