Infants, children and young people with life-limiting or life-threatening conditions often experience acute, transient pain episodes known as breakthrough pain. There is currently no established way to assess breakthrough pain in paediatric palliative care. Anecdotal evidence suggests that it is frequently underdiagnosed and undertreated, resulting in reduced quality of life. The development of a standardised paediatric breakthrough pain assessment, based on healthcare professionals' insights, could improve patient outcomes. This study aimed to explore how healthcare professionals define and assess breakthrough pain in paediatric palliative care and their attitudes towards a validated paediatric breakthrough pain assessment. This was a descriptive qualitative interview study. Semi-structured interviews were conducted with 29 healthcare professionals working in paediatric palliative care across the UK. An inductive thematic analysis was conducted on the data. Five themes were generated: 'the elusive nature of breakthrough pain', 'breakthrough pain assessment', 'positive attitudes towards', 'reservations towards' and 'features to include in' a paediatric breakthrough pain assessment. The definition and assessment of breakthrough pain is inconsistent in paediatric palliative care. There is a clear need for a validated assessment questionnaire to improve assessment, diagnosis and management of breakthrough pain followed by increased healthcare professional education on the concept.