Endotracheal suction (ETS) is a common method used to remove broncho-pulmonary secretions. This study aimed to investigate whether, and how, nurses in the paediatric intensive care environment assess the need for endotracheal suction. Overt, non-participant observation and interviews were used to collect data from 12 nurses on four paediatric intensive care units. The large majority of episodes of ETS observed were performed in response to an identified clinical need based on changes in a variety of clinical signs and patient behaviour. This study highlights the complex nature of nurses’ assessment of the need for ETS and has implications for training and education of nursing staff.