The endocrine system functions as a communicator and an effector. It allows 'action at a distance' in response to changes in the external and internal environment: communicating a change and effecting a response through the actions of hormones. There is reciprocity in the relationship between the endocrine and metabolic responses to critical illness. The endocrine system drives change while, in turn, the metabolic responses impact on hormone production and action. Critical illness constitutes a severe pathophysiological stressor that elicits a diverse though characteristic pathophysiological response. Patients with critical illness are exposed to the very frontier of medical technology. They are assessed thoroughly, progress is monitored closely, and outcomes are measured accurately. Despite the heterogeneity of critical illness, it is clear we must engage with it as a legitimate field of study if we are to resolve our uncertainties over the significance of endocrine and metabolic end points.