In the pediatric population, defects in adrenal steroidogenesis are the main cause of primary adrenal insufficiency (AI) whereas sudden withdrawal of chronic glucocorticoid therapy is the main cause of central AI. Neonatal screening programs for congenital adrenal hyperplasia are of vital importance, as they allow for an early diagnosis and implementation of appropriate therapeutic interventions to ensure the survival of affected neonates. Baseline serum cortisol concentrations below 3μg/dl are diagnostic of AI, while values above 15–18μg/dl would rule it out. In case of indeterminate values, dynamic assessment of adrenal reserve would be indicated. A short stimulation test with 250μg of 1-24-ACTH is the gold standard in patients with primary AI and is also recommended for evaluating patients with central AI. An insulin-induced hypoglycemia test is the gold standard for evaluating AI of central origin.