Given a complete Riemannian manifold M and a Schrödinger operator − + m acting on L p (M), we study two related problems on the spectrum of − +m. The first one concerns the positivity of the L 2 -spectral lower bound s(− + m). We prove that if M satisfies L 2 -Poincaré inequalities and a local doubling property, then s(− + m) > 0, provided that m satisfies the mean condition inf p∈Mfor some r > 0. We also show that this condition is necessary under some additional geometrical assumptions on M.The second problem concerns the existence of an L p -principal eigenvalue, that is, a constant λ ≥ 0 such that the eigenvalue problem u = λmu has a positive solution u ∈ L p (M). We give conditions in terms of the growth of the potential m and the geometry of the manifold M which imply the existence of L p -principal eigenvalues.Finally, we show other results in the cases of recurrent and compact manifolds.