Secondary adrenal insufficiency (SAI) is a clinical disorder that results from hypothalamic or hypophyseal damage or from prolonged administration of supraphysiological doses of glucocorticoids. Since glucocorticoids are widely used for a variety of diseases, the prevalence of SAI is by far exceeding that of primary adrenal insufficiency. Although the presentation of adrenal insufficiency may be insidious and difficult to recognize, an appropriate adrenocortical hormone replacement could lead to a normal quality of life and longevity can be achieved. The spectrum of adrenal insufficiency ranges from overt adrenal crises to subtle dysfunctions in asymptomatic patients who may be at risk of developing acute adrenal insufficiency since their hypothalamic-pituitary-adrenal (HPA) axis cannot appropriately react to stress. Thus, identification of patients with subtle abnormalities of the HPA is mandatory for avoiding this life–threatening event in stressful conditions. The optimal tests and the optimal testing sequence for adrenal insufficiency are still matter of debate. Insulin tolerance test (ITT) could be the gold standard, as it tests the whole HPA axis, but there are some patients who pass the ITT failing the ACTH test. Various alternatives to the ITT, including the standard cosyntropin stimulation test (SST) and low-dose SST, have been proposed since the adrenal gland in SAI loses the capacity for a prompt response to ACTH stimulation. The standard ACTH dose, but not the 1 μg dose, increases adrenal blood flow and this may contribute to produce an early cortisol response of greater magnitude. Moreover, the loss of the early cortisol response to ACTH stimulation could be a specific property of adrenal insufficiency, thus being a sensitive and early marker of failing adrenal function. While the results of the SSTs are often positive in patients with long-standing and severe disease, in patients with mild or recent-onset SAI these tests, using either 250 μg or 1 μg ACTH, tend to give normal results; thus, a negative cosyntropin test result does not rule out the possibility of SAI. Further studies with a systematic comparison of the different tests in large series of patients submitted to a prolonged follow-up are needed to solve the controversy of the optimal diagnostic strategy of SAI.