  Neutrino oscillation physics has entered a new precision era, which poses
major challenges to the level of control and diagnostics of the neutrino beams.
In this paper, we review the design of high-precision beams, their current
limitations, and the latest techniques envisaged to overcome such limits. We
put emphasis on "monitored neutrino beams" and advanced diagnostics to
determine the flux and flavor of the neutrinos produced at the source at the
per-cent level. We also discuss ab-initio measurements of the neutrino energy
-- i.e. measurements performed without relying on the event reconstruction at
the neutrino detector -- to remove any flux-induced bias in the determination
of the cross sections.
