The problem of the characterization of all analytic potentials which give rise to isochronous oscillatory motions still open. However, there are several approaches to highlight motions with period $T(E) \equiv T_0$ independent on the energy. It is proposed in this paper to give necessary and sufficient conditions for a center to be isochronous. The proofs produced here are self contained. As corollaries we find again lot of known conditions previously established by different authors. This allows us to produce several class of isochronous analytic motions. We are also interested in the quantum spectrum. We then use the perturbation method WKB to derive an expression for the corrections to the equally spaced valid for analytic isochronous potentials.