Existing thermal barrier coatings (TBCs) can be adapted enhancing their functionalities such that they not only protect critical components from hot gases, but also can sense their own material temperature or other physical properties. The self-sensing capability is introduced by embedding optically active rare earth ions into the thermal barrier ceramic. When illuminated by light the material starts to phosphoresce and the phosphorescence can provide in-situ information on temperature, phase changes, corrosion or erosion of the coating subject to the coating design. Quantitative measures of the degradation can be collected by non-destructive means such that the deterioration could be monitored. The results from different applications of the technology illustrate how the techniques could be used to gather information to move towards predictive maintenance routines. In addition, sensor TBCs can be employed to conduct temperature measurements either during operation or off-line to optimise operating conditions and reliably move to more efficient engine operation. Temperature measurements have been demonstrated during operation of a gas turbine. Furthermore, coatings have been installed in a UK power station and the profile of the luminescence decay time indicates the operating temperature map which corroborated with micrograph analysis.