Gravitational Waves and the Polarisation of the Cosmic Microwave Background

We discuss the influence of gravitational waves (GWs) upon the polarisation of the Cosmic Microwave Background Radiation (CMBR). We show how to compute the {\em rms} temperature anisotropy and polarisation of the CMBR induced by GWs of arbitrary wavelength. We find that the ratio of polarisation, $\mathrm{\Pi }$, to anisotropy, $A$, can be as large as $\sim 40$%, but is sensitively dependent upon the GW spectrum and the cosmological ionisation history. We argue that CMBR polarisation measurements can provide useful constraints on cosmological models.