The aim of this chapter is to provide nurses with the knowledge to be able to assess, manage, and care for people with type 1 and type 2 diabetes mellitus in an evidence-based and person-centred way. Diabetes mellitus is a long-term condition that can affect people of all ages; consequently, people with diabetes mellitus can be found in every healthcare environment, from hospitals to care homes. The chapter will provide a comprehensive overview of the classifications, causes, and risk factors of diabetes. The key principles of patient assessment are established, before exploring best practice to deliver care, prevent acute complications, and minimize long-term complications. Nursing assessments and priorities are highlighted throughout, and the nursing management of the symptoms and common health problems associated with diabetes can be found in Chapters 19, 22, 24, 25, 26, and 28, respectively. Diabetes mellitus is a group of metabolic conditions with hyperglycaemia occurring as the main feature. It is characterized by chronic increased blood glucose (hyperglycaemia), with disturbance of carbohydrate, protein, and fat metabolism, which results from defects in insulin secretion, insulin action, or both (World Health Organization (WHO), 1999). The hormone insulin, produced by the beta cells in the pancreas, controls blood glucose levels, keeping them within a narrow range in normal health (4–6 mmol/l before food). When blood glucose levels rise (for example, after a meal containing carbohydrates has been consumed), glucose enters the beta cells, eventually resulting in the release of insulin into the portal circulation. The classifications of diabetes mellitus (World Health Organization, 2006) are as follows….● Type 1 diabetes mellitus, previously known as insulin-dependent diabetes mellitus (IDDM) ● Type 2 diabetes mellitus, previously known as non-insulin-dependent diabetes mellitus (NIDDM) ● Gestational diabetes mellitus ● Others, such as disorders affecting the pancreas, and endocrine conditions…The features of type 1 and type 2 diabetes mellitus are outlined in Table 9.1. Gestational diabetes is carbohydrate intolerance, resulting in hyperglycaemia with onset or recognition during pregnancy (World Health Organization, 2006). However, the condition may have been present prior to pregnancy, but not been diagnosed. Diabetes mellitus may occur for other reasons, including genetic defects and diseases affecting the pancreas.