- Engineering thermoplastics, in particular polyolefins such as special grades of poly(ethylene), are gaining importance in pipe applications such as gas and water supply systems. To ensure proper performance of such pipes over the required lifetime, polymer physics and mechanics concepts are needed to adequately account for the effects of time, temperature, and environmental conditions as well as the occurrence of pipe defects and imperfections on relevant polymer properties and pipe performance. This article provides a critical overview of the scientific background of current methodologies to describe the long-term behavior of thermoplastic pressure pipes. In particular, the merits and limitations of two different approacnes - namely, the standard extrapolation method (SEM) described in ISO/TR 9080 and the linear elastic fracture mechanics (LEFM) approach - are compared. Special attention is given to effects associated with material ageing and degradation.