Interdiffusion can be a major cause of failure in coated parts that see service at elevated tempera- tures. Ways to measure the extent of interdiffusion and mathematical equations for predicting these meas- ures are given. The equations are based on the error function solution to the diffusion equation and do not take into account variations of the diffusivity with composition. Also, when the substrate of the coating is multiphase, the equations do not take into account the precipitate morphology, but do take into account that precipitates can act as sinks or sources of solute as the average composition of the substrate varies. The equations are meant to be alloy design tools that indicate how changing substrate or coating chemistry will reduce the extent of interdiffusion. Interdiffusion can be a major cause of failure in coated parts that see service at elevated tempera- tures. Ways to measure the extent of interdiffusion and mathematical equations for predicting these meas- ures are given. The equations are based on the error function solution to the diffusion equation and do not take into account variations of the diffusivity with composition. Also, when the substrate of the coating is multiphase, the equations do not take into account the precipitate morphology, but do take into account that precipitates can act as sinks or sources of solute as the average composition of the substrate varies. The equations are meant to be alloy design tools that indicate how the substrate or coating chemistry will reduce the extent of interdiffusion.