Given a bounded open subset Ω of R n , we establish the weak closure of the affine ball B A p (Ω) = {f ∈ W 1,p 0 (Ω) : Epf ≤ 1} with respect to the affine functional Epf introduced by Lutwak, Yang and Zhang in [43] as well as its compactness in L p (Ω) for any p ≥ 1. This part relies strongly on the celebrated Blaschke-Santaló inequality. As counterpart, we develop the basic theory of p-Rayleigh quotients in bounded domains, in the affine case, for p ≥ 1. More specifically, we establish p-affine versions of the Poincaré inequality and some of their consequences. We introduce the affine invariant p-Laplace operator ∆ A p f defining the Euler-Lagrange equation of the minimization problem of the p-affine Rayleigh quotient. We also study its first eigenvalue λ A 1,p (Ω) which satisfies the corresponding affine Faber-Krahn inequality, this is that λ A 1,p (Ω) is minimized (among sets of equal volume) only when Ω is an ellipsoid. This point depends fundamentally on PDEs regularity analysis aimed at the operator ∆ A p f . We also present some comparisons between affine and classical eigenvalues and characterize the cases of equality for p ≥ 1. All affine inequalities obtained are stronger and directly imply the classical ones.