An organism is a dynamic system, and its life history results from underlying processes in time. The effects of biological and chemical stressors on this system therefore also change temporally. In the present short communication, we emphasize the importance of including time as a factor in stress ecology and ecotoxicology and argue that current standard test protocols may benefit considerably from this, improving data interpretation and thus also risk assessment and risk management. Environ. Toxicol. Chem. 2010;29:1396-1398. © 2010 SETAC.