1. 1. Invasive versus Non-Invasive Clinical Measurements in Medicine Clinical measurement has become an essential complement to traditional physical diagnosis. An ideal clinical measurement should be quantitative, have a high level of reliability and accuracy, be safe, acceptable to the patient, easy to perform and non-invasive. The latter demands that the technique should not break the skin or the lining epithelium and should be devoid of effects on the tissues of the body by the dissipation of energy or the introduction of infection [1]. It is therefore logical that for a given measurement, a non-invasive test will be preferred if it provides the same information with the same accuracy and precision. In the following sections, we will discuss the role of various non-invasive or relatively non-invasive methods to assess airway inflammation in asthma and concentrate on the only direct method of induced sputum examination. 1. 2. Why Is Assessment of Airway Inflammation Important in Asthma? Inflammation is a localized protective response elicited by injury or destruc- tion of tissues which serves to destroy, dilute or wall off both the injurious agent and the injured tissue [2]. The role of inflammation in asthma was rec- ognized long ago. In his textbook The Principles and Practice of Medicine, in 1892, Sir William Osler described "bronchial asthma . . . in many cases is a spe- cial form of inflammation of the smaller bronchioles . . .