  Beam polarisation is an integral part of the physics case of future Linear
Colliders. In this contribution, important examples from Higgs coupling
measurements, top and electroweak physics at high energies, the Z pole program
as well as from searches for production of new particles will be reviewed. The
full exploitation of its advantages requires the polarisation to be known at
the permille-level. The a polarimetry concept based on the combination of
Compton polarimeters, spin-tracking simulations and a global analysis of
collision data which has been developed for the ILC to achieve the required
precision will be presented.
