  We discuss the impact of near detectors at a neutrino factory both on
standard oscillation and non-standard interaction measurements. Our systematics
treatment includes cross section errors, flux errors, and background
uncertainties, and our near detector fluxes include the geometry of the
neutrino source and the detector. Instead of a specific detector concept, we
introduce qualitatively different classes of near detectors with different
characteristics, such as near detectors catching the whole neutrino flux (near
detector limit) versus near detectors observing a spectrum similar to that of
the far detector (far detector limit). We include the low energy neutrino
factory in the discussion. We illustrate for which measurements near detectors
are required, discuss how many are needed, and what the role of the flux
monitoring is. For instance, we demonstrate that near detectors are mandatory
for the leading atmospheric parameter measurements if the neutrino factory has
only one baseline, whereas systematical errors partially cancel if the neutrino
factory complex includes the magic baseline. Finally, near detectors with
nu_tau detection are shown to be useful for non-standard interactions.
