Let $(M,g)$ be a closed Riemannian manifold of dimension $n \ge 3$ and $P_g$ be a conformally-covariant operator on $(M,g)$. We consider in this talk two problem at the crossroads of conformal geometry and spectral theory: 1) determining the extremal value that the renormalized eigenvalues of $P_g$ take as $g$ runs through a fixed conformal class and 2) determining whether these extremal values are attained at an extremal metric. Examples of such operators $P_g$ include the famous conformal Laplacian of the Yamabe problem, $P_g = \Delta_g + c_n S_g$, but also its higher-order generalisations such as the GJMS operators of order $2k$ for any positive integer $k$.