  The efforts to improve on the precision of the measurement and theoretical
prediction of the anomalous magnetic moment of the muon a_mu have turned into a
test of our understanding of the hadronic contribution to vacuum polarisation.
I describe how recent measurements of hadron production in e+e- interactions
with initial-state radiation provide precision measurements of the hadron cross
section, and have improved on the contribution to the prediction of the value
of a_mu that dominates the global uncertainty.
