When addressing the question of whether or not the respiratory muscles are weak, and quantifying exactly how severe any weakness might be, it is sensible to regard the tests available as complimentary, and to proceed by using simple tests first and invasive tests only when appropriate. This sequential and complimentary approach is illustrated in table 1. Thus, if the supine vital capacity is normal, it is very unlikely that there is clinically significant respiratory muscle weakness. However, if the vital capacity is low, it may then be appropriate to measure the maximal inspiratory and maximal expiratory pressures. If the mouth pressures are normal, weakness has been excluded, but if the pressures are low it will then be necessary to proceed to the next tests on the list. Thus, the process continues and, when all of the tests have been performed, a comprehensive description can be achieved of any respiratory muscle weakness that is present. We now have the capacity, if we so wish, to measure respiratory muscle weakness accurately. The challenge is to use these techniques to demonstrate the undoubted importance of respiratory muscle weakness in a wide variety of clinical circumstances.