The aim of this qualitative study was to gain a better understanding of how nurses working on inpatient specialist palliative care units assess and manage breakthrough pain. Thematic analysis of semi-structured interviews with fifteen nurses from five different specialist palliative care units in the UK was undertaken. Themes identified have been broadly categorized into four main areas: defining breakthrough pain, assessing breakthrough pain, managing breakthrough pain, and attitudes/teamwork. Nurses had difficulty defining breakthrough pain as a distinct pain subtype and were often unable to differentiate it from poorly controlled background pain. This study highlights significant training needs and suggests that the theoretical work and recently published consensus recommendations around breakthrough pain now need to be translated into day-to-day clinical practice.