Pain has many valuable functions. It can be a warning or force us to rest our bodies. Yet most ongoing chronic pain, such as unrelenting backache or headache, has no discernable cause and diminishes countless lives. Over the years a scientific revolution has taken place in chronic pain research and therapy. A major catalyst for this was the introduction of the `gate theory' by Professor Ronald Melzack and Professor Patrick D. Wall, which argued that pain is a unified stream of experience generated by the brain, incorporating a whole host of psychological functions. Their now-classic book, with a new introduction taking in all the latest medical developments, examines every facet of pain: the psychological and clinical aspects, the physiological evidence, the major theories of pain and the developments in its control. The challenge in the twenty-first century is to look at how memories, personal and social expectations, genetics, gender, aging and stress patterns all play a role in pain, and how understanding this could lead to the relief of the suffering endured by millions.