The status of the measurement of the muon magnetic moment and of its prediction within the Standard Model is reviewed. Both approaches have been followed for decades with ever increasing precision.Their relative progress is a necessity in order to test the present theory and possibly find hints for new physical phenomena. Indeed, the recent measurement from Fermilab shows a tantalizing 4.2 standard-deviation excess over the up-to-date prediction. Particular emphasis is placed on the contribution from hadronicvacuum polarization which controls the theoretical precision of the prediction. It is
evaluated through a dispersion relation using as input experimental results on electron-positron annihilation into hadrons which are thus playing a key role in establishing the prediction.