The CMB is already one of the pillars of the Big Bang model .
However it may also become our most powerful tool to distinguish contending models and to determine their cosmological parameters .
To realize this goal , more than 20 observational groups and two new satellites are gearing up to make precise measurements of the CMB at small angular scales .
In such a situation it is important to keep track of what the CMB data can already say about cosmological parameters .

Current CMB data can already be used to constrain cosmological parameters .
The results are model dependent .
We have obtained contraints on Hubble ’ s constant h and the density of the Universe \Omega _ { o } in the context of open and critical density CDM models with \Lambda = 0 .
In critical density models we obtain h = 0.30 ^ { +0.18 } _ { -0.07 } .
This low value is inconsistent with direct measurements of h but fully consistent with four other cosmological measurements : Big Bang nucleosynthesis , cluster baryonic fraction , age constraints from globular clusters and limits on the shape parameter \Gamma of matter power spectra ( in \Omega _ { o } = 1 models ) .
If \Omega _ { o } is left as a free parameter the constraints on h are less restrictive : h = 0.40 ^ { +0.57 } _ { -0.14 } .
This is fully consistent with local h measurements and the four other cosmological measurements mentioned above .
The best-fit density is \Omega _ { o } = 0.85 and we set an upper limit of \Omega _ { o } > 0.4 at \sim 95 \% CL .

At this conference Ostriker has claimed that open-CDM models with \Omega _ { o } \approx 0.3 and h \approx 0.70 are compatible with all current data .
However our new CMB data analysis rules this model out at more than \sim 4 \sigma .