The aim of this document is to provide guidance on the optimal activity of Neonatal Intensive Care Units (NICUs) in the UK. Optimal is defined as providing a combination of the lowest mortality and morbidity, the best cost effectiveness and the best baby and parent experience. A NICU is defined as that described in the Department of Health (DH) Toolkit for Neonatal Services which equates to Level 3 Neonatal Intensive Care Units in accordance with the international classification. Medical staffing is defined as roles traditionally undertaken by medical practitioners but including those which can now also be undertaken by appropriately trained and experienced Advanced Neonatal Nurse Practitioners (ANNPs). Intensive Care days, High Dependency days and Special Care days are defined as described in the BAPM Categories of Care 2011.