  The leading-order hadronic contribution to the muon magnetic anomaly a_{\mu}
= (g_{\mu} - 2)/2, calculated using a dispersion integral of e+e- annihilation
data and tau decay data, is briefly reviewed. This contribution has the largest
uncertainty to the predicted value of a_{\mu}, which differs from the
experimental value by ~3.6 (2.4) standard deviations for the e+e- (tau) based
analysis. New results since the last workshop and main open issues on the
subject are discussed.
