Over the past 10 years, several papers have been published discussing the long-term mechanical durability of the cement sheath. The customary procedure is to use a model to predict potential failure scenarios and to subsequently design a sealant material that will not fail under the expected conditions. The predictive models are either analytical or finite-element models. The analytical models can only be applied to relatively simple situations that require a simplified set of input data. In these cases, the results are consistent with those of finite-element models. More complex situations can be simulated with finite-element models, but the input data requirements are far greater. Typically in the modeling papers, little information is included on how the input data is obtained. Because of this, several papers have been published that have proposed ways to obtain the input data, in particular the mechanical parameters of the set cement. Because typically these papers have addressed only one or two parameters however, the proposed methods are inconsistent. This paper critically reviews the published information and highlights the strengths and weaknesses of the previous approaches. Subsequently, the paper presents new measurement methods and data analysis techniques to determine cement mechanical parameters in sufficient detail to allow them to be implemented in any laboratory with appropriate equipment. The predictions using data from the new measurement methods have been verified at the field level through evaluation of actual wells. Finally, the methods have been used to demonstrate the mechanical durability of flexible cement systems aged at high temperatures for one year; this is the first time that such data have been presented on these systems.