After 40 years, the Glasgow Coma Scale (GCS) is the resource of choice for assessing the level of consciousness in patients with neurological conditions. Clinicians’ ability to monitor patients’ conditions, identify deterioration and make clinical decisions depends on their ability to carry out GCS assessments, so it is vital that they understand it. This article explores how best to use the GCS in clinical practice and examines some of the factors that can affect the accuracy of assessments. The article also explains the difference between peripheral and central stimuli.