ABSTRACT This paper reviews the experience at Jaguar Land Rover (JLR) of developing cooling system performance sign-off criteria for hybrid powertrain vehicles. The paper explores customer expectations for electric vehicle mode operation in extreme hot and cold environments and describes a process to assist in determining the upper and lower ambient temperature thresholds to ensure customer satisfaction. A method to assist in the development of new transient thermal sign-off drive cycles for hybrid power trains is proposed, together with examples of how this can be applied to typical hybrid driving scenarios. The role of CAE methods in this process and examples of the key considerations are also discussed. Example test results are reviewed which show the extent to which the new hybrid cooling sign-off criteria developed at JLR has achieved its objectives.