In modelling mass and heat transfer steps in metallurgical processes, it is important to have knowledge of the physical properties of slags, the most important among these being the surface and interfacial tensions, thermal diffusivities, optical properties and viscosities. A critical review is presented of work reported in the past two decades relating to the following properties of slag systems: (i) surface/interfacial tensions and related interfacial phenomena; (ii) thermal diffusivities and thermal conductivities; (iii) velocities and coefficients of absorption of ultrasonic waves; (iv) optical properties. A perspective for further work is also provided.