Sharp comparison theorems are derived for all eigenvalues of the weighted Laplacian, for various classes of weighted-manifolds (i.e. Riemannian manifolds endowed with a smooth positive density). Examples include Euclidean space endowed with strongly log-concave and log-convex densities, extensions to pexponential measures, unit-balls of ℓ n p , one-dimensional spaces and Riemannian submersions. Our main tool is a general Contraction Principle for "eigenvalues" on arbitrary metric-measure spaces. Motivated by Caffarelli's Contraction Theorem, we put forth several conjectures pertaining to the existence of contractions from the canonical sphere (and Gaussian space) to weighted-manifolds of appropriate topological type having (generalized) Ricci curvature positively bounded below; these conjectures are consistent with all known isoperimetric, heat-kernel and Sobolev-type properties of such spaces, and would imply sharp conjectural spectral estimates. While we do not resolve these conjectures for the individual eigenvalues, we verify their Weyl asymptotic distribution in the compact and non-compact settings, obtain non-asymptotic estimates using the Cwikel-Lieb-Rozenblum inequality, and estimate the trace of the associated heat-kernel assuming that the associated heat semi-group is hypercontractive. As a side note, an interesting trichotomy for the heat-kernel is obtained. so that the usual integration by parts formula is satisfied with respect to µ:Here ∇ g denotes the Levi-Civita connection, ∆ g denotes the usual Laplace-Beltrami operator, and we use ·, · = g. One immediately sees that −∆ g,µ is a symmetric and positive semi-definite linear operator on L 2 (µ) with dense domain C ∞ c (M ), the space of compactly supported smooth functions on M . In fact, it is well-known (e.g. [8, Proposition 3.2.1]) that the completeness of (M, g) ensures that −∆ g,µ is essentially self-adjoint on the latter domain, and so its graph-closure is its unique self-adjoint extension. We continue to denote the resulting positive semi-definite self-adjoint operator by −∆ g,µ , with corresponding domain Dom(∆ g,µ ). By the spectral theory of self-adjoint operators (see Subsection 3.2), the spectrum σ(−∆ g,µ ) is a subset of [0, ∞). When the spectrum is discrete (such as for compact manifolds), it is composed of isolated eigenvalues of finite multiplicity which increase to infinity; we denote these by λ k = λ k (M n , g, µ), and arrange them in non-decreasing order (repeated by multiplicity) 0 ≤ λ 1 ≤ λ 2 ≤ . . .. In the discrete case, when M is connected and µ has finite mass, we always have 0 = λ 1 < λ 2 . For the standard definition of {λ k } when the spectrum is possibly non-discrete (as the first eigenvalues until the bottom of the essential spectrum), we refer to Subsection 3.2. In this work, we would like to investigate the spectrum of various classes of weighted-manifolds.Definition. The weighted-manifold (M n , g, µ) satisfies the Curvature-Dimension condition CD(ρ, N ), ρ ∈ R and N = ∞, if: Ric g,µ := Ric g + ∇ 2 g V ≥ ρ g (1.1)as...