Cancer has been the top one cause of death in Taiwan for more than 20 years since 1982. Pain is one of the most prevalent and dreadful complications in these patients. Although there were many available analgesics of different mechanisms, the control of cancer pain still has room for improvement. Pain sensation is transduced, transmitted, modulated, and integrated through complex system. Opioid receptors are one of the major players in the complex system and opioids, therefore, are the most commonly prescribed agents in cancer pain management. This article introduces the mechanism of pain sensation, the types of cancer pain, the principles in managing cancer pain through opioids, and the approaches to relieve the commonly seen adverse effects.