Chronic cerebrospinal venous insufficiency (CCSVI) is a term used to describe impaired venous drainage from the central nervous system (CNS) caused by abnormalities in anatomy and flow affecting the extracranial veins. Recently, it has been proposed that CCSVI may contribute to the pathogenesis of multiple sclerosis (MS). It is hypothesized that venous obstruction results in abnormal flow that promotes inflammation at the blood-brain barrier and that this triggers a process marked by a disturbance of homeostasis within the CNS that leads to demyelination and neurodegeneration. The venous abnormalities of CCSVI are often diagnosed by ultrasound or magnetic resonance venography, however the prevalence of CCSVI detailed in groups of MS patients and patients without MS varies widely in published reports. Increased standardization of diagnostic studies to evaluate both anatomical and physiological findings associated with CCSVI is needed. The purpose of this article is to provide a background to understand the development of the theory of CCSVI and to frame the relevant issues regarding its diagnosis and relationship to the pathogenesis of MS.