Natural history data increase the descriptive knowledge about amyotrophic lateral sclerosis (ALS), help define primary outcome variables and numbers of patients needed on clinical trials, and may give valuable predictive information. Global scores do not adequately represent the clinical variability of ALS. The Charing Cross Quantitative and Qualitative ALS Rating Scales assess disease severity and progression by validated regional scores (bulbar, respiration, upper limb, lower limb) and activities of daily living. The main stages in the development of these scales are summarized. Interval, or quantitative, scales provide accurate and sensitive measurements of the evolution of the disease and are useful for phase II therapeutic trials. The deterioration rates of regional scores in individual patients may not be linear. Rates of disease progression in ALS vary (1) among patients, (2) among topographical regions within a single patient, and (3) at different stages of the disease in a single region in the same patient. The deterioration rates of the regional scores of an ALS population depend critically on whether deaths are included or excluded from the population mean scores. Qualitative scales with simple scores are best suited for large-scale, phase III trials and for life table analysis of times to failure.