When treating patients with pain, it is important to at least try to understand what pain is. This first article in a series on chronic pain covers some general aspects of pain and its taxonomy, in particular the differences between acute and chronic pain, and between nociceptive and neurogenic pain. The next article will explore the psychology of pain, and in particular the relationships between pain, anxiety and depression. A follow-up article will be devoted to some of the thorny issues of pain measurement. The references given should be useful as a starting point for anyone embarking on their own exploration of the literature on chronic pain.