Gas turbine engines operate under the conditions of high temperature and stresses, and the engine components are also subjected to corrosion, oxidation and abrasion. Ceramic Thermal barrier coatings (TBCs) when deposited onto the hot sections of turbine engines improve their performance. TBCs have porosity by design, the architecture of which depends on the processing route. The presence of porosity gives rise to its low thermal conductivity and high strain tolerance. The porosity and its architecture can be significantly affected in service, thus changing the properties of the TBC and hence impairing its performance. Understanding the changes in pore architecture and relating it to changes in properties in-service thus forms an important objective. This book, suitable for engineering undergraduates, postgraduates and professionals, therefore, describes the materials' issues involved in the development of ceramic TBCs together with a brief overview of the current TBC systems, their porosity and pore architecture, thermal and mechanical properties, and techniques available for measuring these properties.