Patients with the primary progressive form of multiple sclerosis (PPMS) have a unique clinical course and demonstrate additional demographic and imaging features, which separate them from the relapsing/remitting form of the condition. Whether these features indicate a fundamental difference in the underlying pathogenesis of the condition or simply reflect opposite ends of a clinical spectrum is unclear. What is clear, however, is that this form of MS provides a valuable model of progression, which has the potential to explain this most disabling component of the disease process. The lack of the hallmark relapses and remissions in PPMS poses diagnostic difficulties, some of which have been addressed by recently published diagnostic criteria. Following diagnosis, the need for information, specific to this form of MS, must be recognized and addressed.