Thoracic surgery ranges from small low-risk procedures to major surgery, and for malignant and non-malignant disease. Assessment of the thoracic patient for surgery comprises two distinct areas. The first is the resectability of the lesion if malignant and the second is the fitness to withstand the morbidity it inevitably involves, referred to as operability by most surgeons. Lung function tests described in the chapter include spirometry, gas transfer capacity, functional tests, and arterial blood gas analysis. Sensitivity for detecting small lesions is reasonable but not as high as computerized tomography (CT) scans but the radiation exposure is very low and the investigation is widely available. In general, the resolution of the magnetic resonance imaging (MRI) scan is not superior to the CT scan and it is no better at confirming the presence or absence of invasion than CT scanning.