Airways are conduits for air between the environment and the alveoli. Pathological events in the airways bring about uneven distribution of gas in the alveoli and increased breathing work. The goals of this chapter are as follows: one, to recognize pathological changes in the airways; two, to recognize the physiological changes accompanying these pathological changes and to understand the conceptual basis for these changes; and three, to become familiar with the test procedures available to the clinician for the evaluation of airway function. This chapter reviews measurements of flow, volume, and pressure and assesses the strengths and limitations of the commonly used tests of dynamic lung function.