The pathophysiology of lower back pain is a source of fascination, frustration, and, often, confusion by clinicians and scientists who both treat and study patients who have this problem. The statement that we do not know the origin of low back pain in the majority of cases has become a mantra to any introduction to the topic. The Agency for Health Care Policy and Research (AHCPR) Guidelines on Low Back Problems and similar guidelines in other countries have given up on attempts to define the origin of the pain in patients who have this problem and have instead resorted to differentiating between so-called Red Flag pathologic conditions and patients who have nonspecific low back pain, with or without leg symptoms.