Nurses caring for children need to be able to understand the investigations that contribute to holistic assessment of the child. Blood gas analysis is a common investigation in critical care settings but is also used during stabilisation or preparation for transfer to critical care of children who are acutely ill. In this article the indications for arterial blood gas analysis are discussed and patient groups that may benefit from this investigation identified. The basic science behind acid-base balance and its impact on homeostasis are described. A simple guide to interpreting arterial blood gas results and the principles surrounding care of a child’s arterial line are also addressed.