Increasingly CPET is being used as part of the pre-operative work-up of patients for major surgery. Whilst CPET can not necessarily be used to define a specific risk for a specific patient, it can provide a broad risk stratification which can guide the need for optimisation, prehabilitation, or potential post-operative level of care. It is part of a global assessment of the patient and hard cut-off values should be avoided. A useful mental model is the ‘traffic light model’ (Figure 29.1).