We examine some new aspect of “Quintessence” models, where a minimally-coupled scalar field is thought to be a suitable candidate for the missing energy in the Universe. In particular, we give emphasis on the problem of initial conditions, finding the values of the scalar field and of its first time derivative that realize initial adiabaticity or initial isocurvature. Starting from the corresponding initial conditions, we numerically solve the background equations and the perturbed Einstein equations, in order to obtain the CMB anisotropies spectra and to compare them with that predicted by the ordinary Cold Dark Matter model (Ωm=1). We find characteristic signatures of the scalar field that could be detected by future satellite missions.