Spirometry is a common test in pulmonary outpatient practice. Although the test is relatively straightforward to perform, its interpretation can be surprisingly complex. In this chapter, a methodical approach to interpretation of spirometric results has been presented, and the mechanisms leading to each abnormal parameter along with its own diagnostic relevance have been painstakingly explained. Above all, the approach has been deconstructed and simplified, so that the information dealt with can be assimilated at a single reading.