Data for the viscosity and thermal conductivity of dense gaseous and liquid heavy water (D2O) have been reviewed and critically evaluated. Selected data were fitted to equations, from which tables of values were generated from temperatures up to 500 °C and for pressures up to 100 MPa for the viscosity and up to 550 °C and 100 MPa for the thermal conductivity. The uncertainties of the tabular values were estimated. The present paper is intended to explain the background of the International Representations of the Viscosity and Thermal Conductivity of Heavy Water Substance of the International Association for the Properties of Steam. With the aid of the present correlations, the kinematic viscosity, thermal diffusivity, and Prandtl number have been calculated. The present status of the gaseous diffusion coefficient is also briefly reviewed.