<i>Background:</i> The measurement of neurologic impairment in multiple sclerosis (MS) is of importance in treatment trials and course of illness. <i>Methods:</i> This review describes the rationale underlying the formation and use of a bifid rating system, the (Expanded) Disability Status Scale (DSS) and the Functional Systems (FS). <i>Results:</i> All signs found at neurologic examination in MS can be consolidated into 8 mutually exclusive FS: pyramidal, cerebellar, brain stem, sensory, bowel and bladder, visual, cerebral and other, each of which, save ‘other’, is given ordinal grades from 0 to 5 or 6. Each FS correlates with the DSS, a step scale from 0 (normal) to 10 (death due to MS), which is an overall measure of neurologic abnormality. Dividing steps 1–9 each into 2 gives the Expanded DSS. The FS and DSS were used in an Army series to describe neurologic status at first diagnosis and over the first 20 years of illness. DSS severity at 5 years after onset, but not earlier, was highly predictive of later severity. <i>Conclusions:</i> Combination of the (Expanded) DSS and FS has been used successfully to assess impairment for natural history and treatment studies in MS.