Critically ill paediatric patients are frequently exposed to pain that is required to be assessed and treated effectively. The most reliable resource for assessing pain is the child itself, but children in the paediatric intensive care unit (PICU) are commonly unable to communicate their needs, requiring professional caregivers to uncover and interpret pain. However, nurses and paediatricians do not have sufficient knowledge of how critical illness affects childrens' signs of pain. The aim of this study was to illuminate clinical experiences of pain in the PICU; describing nurses' perceptions of expressions of pain in non-verbal, critically ill 2-6 year old children. The participants were 17 experienced PICU nurses. Data were analysed according to the phenomenographic method and three qualitatively different main categories, gained from clinical experience, emerged: changes in the measurable parameters; perceived muscular tension; and, altered behaviour. Furthermore, contrasting the categories revealed two diverse perspectives to focus pain: measure-oriented and patient-oriented. Subtle expressions of pain were recognised when focus was patient-oriented. These findings support the necessity of actively looking for pain deriving from various perspectives and considering diverse caring needs when doing so. Acknowledging pain makes pain visible.