Rapid Crack Propagation (RCP) is a rare but dangerous failure mode in gas and water pipelines. Although relatively low RCP resistance may be endemic to plastics, extruded polyethylene pipe varies widely in the critical pressure below which the arrest of RCP is ensured at each temperature. The proposed International Standard ‘S4’ pipe test for RCP, and the prediction of S4 and full-scale test critical pressures from basic material data, are discussed. A more effective quality index may the temperature above which and the wall thickness below which RCP becomes impossible at any sustainable pressure. This brittle-tough transition in polyethylene also appears in dynamic fracture resistance tests as a minimum stable crack velocity. Although the well known Irwin-Corten expression can be used to predict critical pressures for notched, water pressurised pipe, the proximity of this transition velocity to that at which the crack driving force in gas-pressurised pipe is a maximum may preclude a similar approach. Charpy-type tests, on sharp-notched specimens of the base resin at varying temperatures and thicknesses, can be used to locate the transition sufficiently closely to index resins. Predicting the RCP performance of pipe extruded from these resins, however, remains uncertain.