Although knowledge of the dielectric properties of water up to very high pressures and temperatures would be essential for many problems of practical importance, laboratory experiments under these conditions are extremely difficult. Here, computer simulations provide a useful alternative. The problems associated with the treatment of long-range interactions have been overcome, and standard present-day technology is sufficient to calculate the dielectric equation of state over a wide range of densities and temperatures. This is demonstrated for SPC/E and two related models of water, in which the effective dipole moment is made densitydependent. The results are also given in the form of an empirical correlation and may be used to assess the validity of extrapolations of experimental data to very high temperatures.