Diabetes mellitus is a chronic disease characterized by high blood glucose levels resulting from defects in insulin production or insulin action. There are two main types - type 1 is caused by lack of insulin and type 2 is caused by insulin resistance. Gestational diabetes occurs during pregnancy and usually resolves after delivery. Diagnosis involves fasting plasma glucose tests or oral glucose tolerance tests. Emergencies can include diabetic ketoacidosis, hyperglycemic hyperosmolar state, or hypoglycemia.