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The LO hadronic vacuum polarization (VP) contribution to the muon anomalousmagnetic moment $a_\mu$ is obtained as the integral as a function of energy ofan expression that involves the ratio of the $e^+e^- \rightarrow\mathrm{hadron}$ cross section to the pointlike muon pair cross section. Theformer is extracted from experimental data for individual hadronic final statesat low energies, and from perturbative QCD at high energies. The BaBarexperiment at SLAC has a programme of systematic measurement of the productionof the lowest-rest-mass hadronic final states, those that contribute mostsignificantly to the integral. To that purpose, we use a method in which, whilethe PEP-II storage ring is operated at a constant energy in the center of masssystem, $\sqrt{s}$, of about 10.6 GeV, events are reconstructed and selectedwhich have been produced with a hadronic final state together with ahigh-energy photon which may (photon tagging) or may not (no tagging) beobserved. In our kinematic configuration the photon is almost always emitted bythe electron or by the positron of the initial state, hence the name"initial-state radiation" (ISR). The cross section for the direct $e^+e^-\rightarrow f$ production of a final state $f$ at an energy $\sqrt{s'}$ is thenextracted from the differential cross section of the ISR production of thestate $f$ with invariant mass $\sqrt{s'}$. The programme is almost completedand has lead to a number of first measurements and to an improvement of up to afactor of three of the uncertainties on the contributions of individualchannels to $a_\mu$.

Low-energy hadronic cross sections measurements at BABAR and g-2 of the muon