Physics and detector studies with the very forward calorimeters at a future linear collider

The very forward region at a future e+ e- collider will be instrumented with calorimeters dedicated primarily for precise measurement of the integral luminosity with the precision required at a percent (CLIC) or at a permille level (ILC). As the constraints on luminosity precision come from the physics programs at future linear colliders, physics and machinerelated systematic effects are discussed as the main sources of uncertainty in luminosity measurement.