Aims : The mechanics of breathing is best described in terms of airways resistance and lung compliance. However, these measurements are not readily available in clinical practice and instead clinicians must focus on the indirect information available from spirometry. The great advantage of spirometry is its known immediate reproducibility and limited between-test variation in both healthy and diseased settings. These properties, coupled with its extensive use in clinical decision making, ensure that spirometry will be used to manage respiratory disease for the foreseeable future. One particular advantage of spirometry is the availability of age- and height-related normal values against which any recording can be judged. This PG course on basic respiratory mechanics will help clinicians to better understand and interpret lung function test results.