Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia due to defects in insulin secretion, insulin action, or both. Chronic hyperglycemia is associated with long-term damage to various organs, including the eyes, kidneys, nerves, heart, and blood vessels. The development of diabetes involves multiple pathogenic processes, ranging from autoimmune destruction of pancreatic β-cells to insulin resistance. The basis of metabolic abnormalities in diabetes is deficient insulin action on target tissues, which can result from inadequate insulin secretion or diminished tissue responses to insulin. Type 1 diabetes is characterized by an absolute deficiency of insulin secretion, often due to autoimmune destruction of β-cells, while type 2 diabetes is caused by a combination of insulin resistance and inadequate compensatory insulin secretion. Other specific types of diabetes include genetic defects in β-cell function, genetic defects in insulin action, diseases of the exocrine pancreas, endocrinopathies, drug or chemical-induced diabetes, infections, and uncommon forms of immune-mediated diabetes. Gestational diabetes mellitus (GDM) is a form of diabetes that occurs during pregnancy and is associated with increased risks for the mother and neonate. The diagnosis of diabetes is based on glucose criteria, including fasting plasma glucose (FPG), 2-hour post-glucose tolerance test (OGTT), and hemoglobin A1C (HbA1C). The American Diabetes Association (ADA) recommends using HbA1C as a diagnostic tool with a threshold of ≥6.5%. The diagnostic criteria for diabetes include FPG ≥126 mg/dl (7.0 mmol/l), 2-hour OGTT ≥200 mg/dl (11.1 mmol/l), and HbA1C ≥6.5%. The diagnosis of GDM is based on a 75-g OGTT at 24-28 weeks of gestation, with specific cutpoints for fasting, 1-hour, and 2-hour plasma glucose measurements. The revised diagnostic criteria for GDM have increased the prevalence of GDM, primarily because only one abnormal value is sufficient for diagnosis. The ADA recognizes the anticipated increase in GDM incidence and is sensitive to concerns about the "medicalization" of pregnancies previously categorized as normal. The diagnostic criteria changes are made in the context of worrisome worldwide increases in obesity and diabetes rates, with the intent of optimizing gestational outcomes for women and their babies. The decision about which test to use for assessing a specific patient for diabetes should be at the discretion of the healthcare professional, taking into account the availability and practicality of testing. The current diagnostic criteria for diabetes are summarized in Table 3.Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia due to defects in insulin secretion, insulin action, or both. Chronic hyperglycemia is associated with long-term damage to various organs, including the eyes, kidneys, nerves, heart, and blood vessels. The development of diabetes involves multiple pathogenic processes, ranging from autoimmune destruction of pancreatic β-cells to insulin resistance. The basis of metabolic abnormalities in diabetes is deficient insulin action on target tissues, which can result from inadequate insulin secretion or diminished tissue responses to insulin. Type 1 diabetes is characterized by an absolute deficiency of insulin secretion, often due to autoimmune destruction of β-cells, while type 2 diabetes is caused by a combination of insulin resistance and inadequate compensatory insulin secretion. Other specific types of diabetes include genetic defects in β-cell function, genetic defects in insulin action, diseases of the exocrine pancreas, endocrinopathies, drug or chemical-induced diabetes, infections, and uncommon forms of immune-mediated diabetes. Gestational diabetes mellitus (GDM) is a form of diabetes that occurs during pregnancy and is associated with increased risks for the mother and neonate. The diagnosis of diabetes is based on glucose criteria, including fasting plasma glucose (FPG), 2-hour post-glucose tolerance test (OGTT), and hemoglobin A1C (HbA1C). The American Diabetes Association (ADA) recommends using HbA1C as a diagnostic tool with a threshold of ≥6.5%. The diagnostic criteria for diabetes include FPG ≥126 mg/dl (7.0 mmol/l), 2-hour OGTT ≥200 mg/dl (11.1 mmol/l), and HbA1C ≥6.5%. The diagnosis of GDM is based on a 75-g OGTT at 24-28 weeks of gestation, with specific cutpoints for fasting, 1-hour, and 2-hour plasma glucose measurements. The revised diagnostic criteria for GDM have increased the prevalence of GDM, primarily because only one abnormal value is sufficient for diagnosis. The ADA recognizes the anticipated increase in GDM incidence and is sensitive to concerns about the "medicalization" of pregnancies previously categorized as normal. The diagnostic criteria changes are made in the context of worrisome worldwide increases in obesity and diabetes rates, with the intent of optimizing gestational outcomes for women and their babies. The decision about which test to use for assessing a specific patient for diabetes should be at the discretion of the healthcare professional, taking into account the availability and practicality of testing. The current diagnostic criteria for diabetes are summarized in Table 3.