The physical properties of mould slags are key to their performance in the continuous casting process. The magnitudes of key properties (viscosity, break temperature, f_cryst and optical properties) are determined by the mould dimensions, casting conditions and the steel grade being cast. However, a range of other properties (e.g. interfacial tension, density) are needed to minimise defects and process problems. The extant data for thermo-physical properties of conventional mould slags are reviewed here and those for specialist powders (e.g. F-free or for casting TRIP steels) are reviewed in Part 2. It was concluded that there is a need for (i) resolution of the huge differences in thermal conductivity of mould slags for T>1050 K obtained with the LP and THW methods (ii) more data for some properties (e.g. C_p and density) and more accuracy for others (viscosity, surface tension) (iii) standardised procedures for the determination of f_cryst and (iv) characterisation of the porosity in slag films. It was also concluded that (i) gaseous convection makes a significant contribution to the heat transfer in the powder bed and (ii) glassy slag films are probably optically-thin.