Cortisol is the major glucocorticoid synthesized by the adrenal cortex and its secretion is under the control of hypothalamic-pituitary axis. There is an increase in corticotrophin-releasing hormone and corticotrophin secretion and a decrease in the negative-feedback effect during critical illness. Adrenal insufficiency (AI) in children with critical illness is defined as an inadequate glucocorticoid response, measured by the peak cortisol or the increment in the cortisol level following exogenous ACTH (corticotrophin) administration. Clinically apparent AI is uncommon in critically ill patients. The incidence of AI in critically ill children varies with the underlying disease, its severity and duration, and multiple different definitions for the diagnosis of AI. Most of the pediatric studies for evaluation of AI during critical illness focused on patients with sepsis or septic shock. In patients with sepsis or septic shock, decreased synthesis or release of corticotrophin-releasing hormone, corticotrophin, and cortisol by cytokines and other circulating mediators released during sepsis are the most likely mechanism of AI. Recent studies in critically ill children reported that the prevalence of AI was not significantly different between septic and nonseptic patients, but it was noteworthy that AI appeared to be common both in septic and nonseptic critically ill children. A multidisplinary approach is necessary to manage to AI in critically ill children. However, no concensus exists among pediatric intensivist and endocrinologysts on diagnosis or treatment of AI in pediatric critical illness.