  A precise prediction of expected neutrino fluxes is required for a
long-baseline accelerator neutrino experiment. The flux is used to measure
neutrino cross sections at the near detector, while at the far detector it
provides an estimate of the expected signal for the study of neutrino
oscillations. In the talk several approaches to constrain the neutrino flux are
presented. The first is the traditional one when an interaction chain for the
neutrino parent hadrons is stored to be weighted later with real measurements.
In this approach differential hadron cross sections are used which, in turn,
are measured in ancillary hadron production experiments. The approach is
certainly model dependent because it requires an extrapolation to different
incident nucleon momenta assuming x_F scaling as well as extrapolation between
materials having different atomic numbers. In the second approach one uses a
hadron production yields off a real target exploited in the neutrino beamline.
Yields of neutrino parent hadrons are parametrized at the surface of the
target, thus one avoids to trace the particle interaction history inside the
target. As in the case of the first approach, a dedicated ancillary experiment
is mandatory. Recent results from the hadron production experiments -
NA61/SHINE at CERN (measurements for T2K) and MIPP at Fermilab (measurements
for NuMI) - are reviewed.
