This chapter looks at the development of chronic pain. It begins by looking at how different types of pain are transmitted: in particular, the role played by peripheral sensitisation and hyperalgesia in transmitting chronic pain. The chapter explores drugs that relieve pain by sensitising the nociceptors and considers the issue of central sensitisation to pain, and examines the problem of peripheral nerve damage. It discusses the role of a range of chemical mediators in transmitting pain. Chronic pain develops when both the sensory neurones and nociceptors become extra sensitive, due to prolonged exposure to inflammatory mediators released during chronic tissue inflammation. In chronic pain the prolonged exposure to chemical mediators and the increase in nerve impulse traffic brings about changes in the expression of genes present in the cell bodies of the nociceptive neurones. Chronic pain is experienced differently from other forms of pain.