  The International Linear Collider (ILC) will collide polarised electrons and
positrons at beam energies of 45.6 GeV to 250 GeV and optionally up to 500 GeV.
To fully exploit the physics potential of this machine, not only the luminosity
and beam energy have to be known precisely, but also the polarisation of the
particles has to be measured with an unprecedented precision of dP/P ~ 0.25%
for both beams.
  An overall concept of high precision polarisation measurements at high beam
energies will be presented. The focus will be on the polarimeters (up- and
downstream of the e+e- interaction point) embedded in the ILC beam delivery
system. Some challenges concerning the design of the Compton spectrometers and
the appropriate Cherenkov detectors for each polarimeter are discussed.
Detailed studies of photodetectors and their readout electronics are presented
focusing specifically on the linearity of the device, since this is expected to
be the limiting factor on the precision of the polarisation measurement at the
ILC.
