  We study the implications on both background and perturbation evolution of
introducing a Chaplygin gas component in the universe's ingredients. We perform
likelihood analyses using wide-ranging, SN1a, CMB and large scale structure
observations to assess whether such a component could be a genuine alternative
to a cosmological constant. We find that current data favors an adiabatic
Chaplygin gas with behavior akin to a cosmological constant.
