We constrain cosmological parameters in flat cosmologies with tracking dark energy ( or Quintessence ) using the existing data on Cosmic Microwave Background ( CMB ) anisotropies .
We perform a maximum likelihood analysis using combined data from COBE/DMR , BOOMERanG , DASI and MAXIMA , obtaining estimates for the dark energy density \Omega _ { Q } and equation of state w _ { Q } , the physical baryon density \Omega _ { b } h ^ { 2 } , the scalar perturbation spectral index n _ { S } , the ratio R between the tensor and scalar perturbation amplitude ( or the tensor spectral index n _ { T } ) .

Dark energy is found to be the dominant cosmological component \Omega _ { Q } = 0.71 ^ { +0.05 } _ { -0.04 } , with equation of state w _ { Q } = -0.82 ^ { +0.14 } _ { -0.11 } ( 68 \% C.L . ) .
Our best fit value of the physical baryon density is in good agreement with the primordial nucleosynthesis bound .
We find no significant evidence for deviations from scale-invariance , although a scalar spectral index slightly smaller than unity is marginally preferred .
Finally , we find that the contribution of cosmological gravitational waves is negligible .

These results confirm that Quintessence is slightly preferred with respect to ordinary cosmological constant by the present CMB data .