Early recognition of patients at risk of catastrophic deterioration leads to early treatment and improved prognosis. Delayed identification of critically ill patients and their delayed referral to intensive care are causes of increased mortality. Systematic assessment of patients at risk is a prerequisite for early recognition of critical illness. The assessment of the critically ill patient should follow the ‘Airway Breathing Circulation’ format. A blocked airway, raised respiratory rate and a systolic blood pressure that is smaller than the heart rate are signs of critical illness. Confusion and raised respiratory rate are the signs that most strongly indicate critical illness and are the most often missed during assessment. The assessment of critical illness is supplemented by risk factors, which result from concomitant chronic conditions, and haematological and biochemical tests, ECGs and radiographs. Lactate has a particular role in triage of patients with sepsis. For those clinicians inexperienced in critical care, simple scoring systems allow rapid assessment of the degree of physiological abnormality and the associated risk.