Epidemiologic research on back disorders is difficult for several reasons. First, there is no standard definition of disc degeneration, so comparisons between studies are difficult because each study defines disc degeneration differently. Second, measures of disc degeneration often lack reliability and precision. Third, accurate measures of lifetime exposures, such as vibration or physical loading, are impossible. Finally, disc degeneration and back pain are not synonymous. Pain is a symptom, whereas the definition of disc degeneration varies with the method used to evaluate the disc; for example, radiographic versus biochemical. Disc degeneration is commonly thought to be associated with severe symptoms and is often targeted diagnostically and therapeutically, but most of the findings associated with disc degeneration, such as desiccation, osteophytes, and disc narrowing, have unclear mechanisms in pain production. Indeed, there is a large body of research into the mechanisms of pain production, but a clear understanding of how the disc causes pain has yet to be gained.