Gestational diabetes mellitus (GDM) is defined as carbohydrate intolerance resulting in hyperglycaemia of variable severity with onset or first recognition during pregnancy. It is a common condition and is associated with increased risk of perinatal complications. This review discusses diagnostic criteria, screening, complications, current management, major studies and randomised controlled trials in GDM and obstetric considerations. Current National Institute for Health and Clinical Excellence (NICE) guidelines, produced by the National Collaborating Centre for Women's and Children's Health and published in 2008 by the Royal College of Obstetricians and Gynaecologists (RCOG) press, are emphasised and controversies highlighted.