Optimizing the glucocorticoid dosage has been a major concern in classic 21OHD (21-hydroxylase deficiency) treatment, as it is essential to adjust it meticulously to the needs of the individual patient. Insufficient glucocorticoid treatment will cause adrenal insufficiency, including life-threatening adrenal crisis, while excess of androgen could cause precocious pubertal growth in children, virilization in female patients, and infertility in male and female adult patients. Meanwhile, overtreatment with glucocorticoids causes iatrogenic Cushing’s syndrome which could result in growth impairment, obesity, osteoporosis, and hypertension. The dilemma of 21OHD treatment is that glucocorticoid supplementation therapy at physiological dosage does not sufficiently suppress ACTH, consequently leading to adrenal androgen excess. Accordingly, the window for the appropriate glucocorticoid treatment would have to be substantially narrower than that of other types of adrenal insufficiency without androgen excess, such as adrenal hypoplasia. For the appropriate management of classic 21OHD, the physician has to be well versed in the physiology of the adrenal cortex, growth, and reproductive function. Comprehensive understanding of patients’ requirements according to their life stage and sex is essential. Furthermore, female patients with 46,XX need to be cared for as differences in sex development (DSD) with careful psychological management. In this review, we aimed to comprehensively summarize the current status of classic 21OHD treatment, including the initial treatment during the neonatal period, management of adrenal insufficiency, maintenance therapy of each life stage, and the importance of clinical management as DSD for 46,XX female patients. The recently developed agents, Chronocort, and Crinecerfont, are also discussed.