It is anticipated that there will be 700 million people with diabetes across the world in 2045 and 5.3 million in the UK by 2025. This chapter explains blood glucose regulation, hypoglycaemia, hyperglycaemia and their symptoms. It describes the long-term complications caused by hyperglycaemic damage to blood vessels, nerves and tissues. These include cardiovascular disease, peripheral arterial disease, ischaemia, diabetic foot disease, eye disease, kidney/renal disease (nephropathy), gum disease (periodontitis), nerve problems (neuropathy), reproductive problems and psychological concerns. The chapter outlines type 1, type 2 and gestational diabetes. It details type 2 diabetes as having non-modifiable risk factors: genetics, ethnicity and age; precursors: prediabetes, metabolic syndrome and oxidative stress/low-grade inflammation; and modifiable risk factors: excess body/abdominal fat, poor diet, physical inactivity, sedentary behaviour, smoking, alcohol, obesogenic environments, traffic, noise and air pollution, persistent organic pollutants and adverse childhood experiences. The chapter concludes by outlining the primary prevention of diabetes, which emphasises healthy weight, healthy eating, physical activity and a healthy environment. It ends with a summary of the secondary and tertiary prevention of diabetes and diabetic complications.