The purpose of this study is to observe the impact leukaemia treatment has on gut health (microbiomes) and how quickly the gut health recovers after leukamia treatment. The gut microbiome has a number of important functions not only in the gut but within the whole body. Changes to your child's nutritional status throughout treatment may affect how well they recover from treatment. This study will monitor the impact of feeding and nutrition on nutritional status and gut health in young people undergoing treatment for leukaemia. The measurements needed to observe nutritional and gut microbiomes will occur when your child attends their routine medical appointments at Great Ormond Street Hospital. Medical treatment uses chemicals to kill leukaemia cells. The type of medications used in the treatment of leukaemia can damage the gut resulting in inflammation call mucositis. This stops the gut from working and sometimes nutrition has to be provided via a feeding tube or intravenous. Chemotherapy, mucositis and intravenous nutrition all have an impact on the gut. Little is know how the gut health recovers after treatment for leukaemia. This will be the first study to specifically monitor the impact of feeding and nutritional on gut health in children undergoing treatment for leukaemia. By understanding what changes are occurring to your child's nutritional status and gut halth during treatment and during recovery will help to develop guidelines for healthcare professionals to support optimal gut health recovery.