Low-back pain is a common ailment. Much is known of the pain-sensitive structures of the low back, but for a patient with acute low-back pain the cause can rarely be defined. Although the biochemical properties of the intervertebral discs and their degenerative changes have been unraveled, it is difficult to distinguish between pathological degeneration and normal changes due to aging. As a consequence of the problems in diagnosing back diseases, the most common measure of low-back morbidity in epidemiologic studies has been the occurrence of symptoms. A wide spectrum of work- and individual-related factors have been found to be associated with low-back pain, but little is known about the extent to which these factors are etiologic and the extent to which they are symptom-precipitating or symptom-aggravating.