Chiari Type I malformation (Chiari I) is a congenital hindbrain anomaly characterized by downward displacement of the cerebellar tonsils through the foramen magnum. This can lead to compression of cerebellar components, the lower brainstem, and the upper cervical spinal cord. In turn, a variety of neurological deficits and permanent nervous system damage may evolve. This review article discusses the etiology, diagnosis, and treatment of patients with Chiari I malformation. Its protean manifestations cause significant overlap with multiple sclerosis, chronic fatigue syndrome, and numerous other conditions. Accordingly, the diagnosis of Chiari I is difficult to establish by clinical evaluation alone. Demonstration of the characteristic hindbrain abnormalities, however, is easily accomplished with magnetic resonance imaging. Neuroimaging should therefore be considered in patients with cerebellar, brainstem, and cervical cord dysfunction. Surgical treatment is indicated in symptomatic patients with radiographic evidence of hindbrain abnormalities. Posterior fossa decompression has also been performed in patients with fibromyalgia and chronic fatigue syndrome based solely on overlapping symptoms with Chiari I. This practice remains controversial. Appraisal of surgical outcome requires postoperative neuroimaging and long-term patient follow-up to assess the permanency of improvement. Preliminary study results of the impact of surgical technique on patient outcome are reported. Ongoing research is devoted to a better understanding of the pathophysiology of Chiari I malformation and the development of more effective medical and surgical treatments.