Breakthrough pain (BTP) is episodic pain that emerges through the treatment of otherwise well-managed chronic background pain. Often called pain flare or transient pain, BTP negatively affects the function and quality of life of the patient and often results in a number of other physical, psychological and social problems. Breakthrough pain is a common occurrence affecting approximately two-thirds of the estimated 50 to 100 million chronic pain sufferers in the US. It can have multiple causes with various pathophysiologies, and can present with numerous clinical features and complications. The clinical features vary from individual to individual, and may vary within an individual over time. The successful management of breakthrough pain depends on proper assessment, treatment, and reassessment. Inadequate assessment can lead to ineffective or inappropriate treatment. Similarly, inadequate reassessment may lead to continuance of ineffective or even harmful treatment. In recent years, the need to educate physicians about pain management has been garnering increased attention from prominent medical associations and the media. Despite ongoing efforts to improve pain treatment, however, the need persists for evidence-based educational materials for physicians in the area of pain diagnosis and management. Part of the Oxford American Pain Library, this highly practical guide covers current approaches and new developments in the assessment and management of breakthrough pain, including both cancer-related pain and non-cancer chronic pain. It addresses the roles of opioid and non-opioid pharmacotherapy and presents non-pharmacologic interventions as well.