The current knowledge of the structures of silicate slags is summarised and the relationships between measures of the depolymerisation of the melt and various physical properties are examined. It is shown that the optical basicity when corrected for the cations required to charge-balance any AlO₄^5- tetrahedra present, provides a reasonable measure of the depolymerisation of the melt and has the advantage over the (NBO/T) ratio that it compensates for cation effects. It is shown that the depolymerisation of the melt is the primary factor affecting most physical properties, the cations having only a secondary effect. The relationships between structure and the viscosity, electrical and thermal conductivity, diffusion coefficient, density, thermal expansion coefficient, thermodynamic and optical properties of melts are discussed.