In his book on behavioural endocrinology, Randy Nelson describes 'stress' as a 'notoriously ethereal concept'. Yet, despite this lack of clarity, studies of the consequences of stress across different time scales, life history stages, taxa and levels of biological enquiry form a large part of modern biology and biomedicine. Organisms need to recognise and respond to environmental challenges. Being able to do so appropriately, and with minimal costs, is an important physiological attribute, with great adaptive value. The costs and benefits of different mechanisms that enable organisms to cope with unpredictable environmental changes can be manifest to different degrees at different life stages. Accordingly, the level of stress experienced in the environment can act as a strong selective pressure that drives the evolution of life histories.