Post-operative pulmonary complications (POPC) occur frequently, especially in patients with pre-existing pulmonary disease and have a significant effect on post-surgical morbidity and mortality. By understanding the patient's existing pulmonary diseases that have a significant effect on post-operative morbidities a combination of information has to be gathered from a thorough history and physical exam as well as selected laboratory and diagnostic tests. Evidence based scores can then be employed to predict the risk of significant POPC. Numbers and testing alone, however, such as diagnosis of chronic obstructive pulmonary disease based on spirometry, may not provide as clear a picture as of the true risk of POPC that is determined by a combination of estimations of the patient's functional status, (b) measured by the patient's estimates of activity and (c) confirmed by the patient's ability to perform simple tasks such as the 6-minute walk test. This information can then be used to rationalize perioperative interventions and improve the safety of the perioperative experience.