X-linked adrenoleukodystrophy (ALD) is caused by mutations in the ABCD1 gene that encodes a protein of the peroxisomal membrane named ALDP. Mutations in ALDP result in elevated levels of very long chain fatty acids (VLCFA) and reduced VLCFA oxidation in peroxisomes. Three main phenotypes are seen in affected males. The childhood cerebral form manifests usually between ages 4 and 8 years. It initially resembles attention deficit disorder or hyperactivity. Progressive central demyelination with impairment of cognition, behavior, vision, hearing, and motor function follow the initial symptoms and often lead to total disability within 2 years. The second phenotype, adrenomyeloneuropathy, manifests most commonly in the late twenties as progressive paraparesis, sphincter disturbances, sexual dysfunction, and often, impaired adrenocortical function; all symptoms are progressive over decades. The third phenotype, 'Addison disease only', presents with primary adrenocortical insufficiency between age 2 years and adulthood and most commonly by age 7.5 years, without evidence of neurologic abnormality. Approximately 50% of females who are carriers develop neurologic manifestations that resemble adrenomyeloneuropathy but have a later onset (age ≥35 years) and a milder disease. In this review, we will give an overview of the present understanding of ALD, and the implications of new diagnostics and treatment.