A principal concern with thermal barrier coatings (TBCs) is their loss of adhesion during service, leading to coating spallation. In this research, an indentation test is used to quantify decreases in interfacial toughness of TBC systems as a function of the duration of isothermal or cyclic high-temperature exposures in dry air or in the presence of water vapor. The indentation test involves penetration of the TBC and the oxide layer below it, inducing plastic deformation in the underlying metal bond coat and superalloy substrate. This plastic deformation induces a compressive radial stress away from the indent, which drives an axisymmetric delamination of the TBC and oxide layers. Test results are presented tracking the "apparent" loss of toughness (that could include changes in the TBC system that contribute to adhesion loss independent of interfacial damage) for EBPVD TBC systems as a function of isothermal exposures from 1100C to 1200C. These results indicate a significant loss in apparent toughness occurs at a fraction of the total TBC system life. Apparent losses in toughness are correlated with observations of increasing oxide thickness and TBC sintering. Further analyses and tests are presented which quantify the relative importance of oxide thickening, TBC sintering and interfacial damage in decreasing apparent TBC system adhesion in these tests. A select group of tests are used to show the importance of as-processed interfacial toughness in determining the rate of toughness loss with exposure and in determining TBC life. Results are also presented that give insight into the importance of steam exposure and cyclic thermal loading, compared to results for isothermal loadings in dry air.