We study Quintessence cosmologies in the context of scalar-tensor theories of gravity, where a scalar field $ϕ$, assumed to provide most of the cosmic energy density today, is non-minimally coupled to the Ricci curvature scalar $R$. Such `Extended Quintessence' cosmologies have the appealing feature that the same field causing the time (and space) variation of the cosmological constant is the source of a varying Newton's constant à la Jordan-Brans-Dicke. We investigate here two classes of models, where the gravitational sector of the Lagrangian is $F(ϕ)R$ with $F(ϕ)=ξϕ^{2}$ (Induced Gravity, IG) and $F(ϕ)=1+ξϕ^{2}$ (Non-Minimal Coupling, NMC). As a first application of this idea we consider a specific model, where the Quintessence field, $ϕ$, obeying the simplest inverse power potential, has $Ω_ϕ=0.6$ today, in the context of the Cold Dark Matter scenario, with scale-invariant adiabatic initial perturbations. We find that, if $ξ\lesssim 5\times 10^{-4}$ for IG and $ξ\lesssim 5\times 10^{-3}(\sqrt{G}ϕ_{0})^{-1}$ for NMC ($ϕ_{0}$ is the present Quintessence value) our Quintessence field satisfies the existing solar system experimental constraints. Using linear perturbation theory we then obtain the polarization and temperature anisotropy spectra of the Cosmic Microwave Background (CMB) as well as the matter power-spectrum. The perturbation behavior possesses distinctive features, that we name `QR-effects', regarding acoustic peak location and height, late time integrated Sachs-Wolfe effect, as well as turnover and amplitude in the matter power spectrum. These features could be detected in the upcoming observations on CMB and large-scale structure.

19 pages including 10 figures, final version to be published in Phys.Rev.D