(1) Pathological or focal activity is that condition of a tuberculous process in which progression of the disease is taking place. (2) The pathology of spreading or actively advancing lesions in pulmonary tuberculosis is discussed. (3) Clinical activity is the clinical expression of pathological activity as manifested by symptoms and physical signs, which, however, may not occur for some time after the onset of pathological activity. (4) X-ray activity can be determined by careful comparison of serial films, and shows up as new areas of soft, fluffy, “cotton-ball” (Sampson) appearance. (5) Blood sedimentation as a test of activity in tuberculosis is discussed, and the Cutler graphic technique is described. (6) Leucocytic reactions, including the lymphocyte-monocyte ratio of Sabin, and the Schilling differential count, are discussed in relation to activity. (7) The results of 168 series of monthly observations of clinical signs and symptoms, roentgenograms, blood sedimentation, and Schilling leucocyte counts in 33 patients with pulmonary tuberculosis, are given and analysed. (8) It is concluded that whilst the order of appearance of manifestations of activity very often is leucocyte changes, then blood sedimentation, then X-ray, and lastly symptoms and physical signs, this order is not invariable. Since no one method has been found to be infallible, none should be relied upon alone. A plea is made that in the determination of activity radiograms and clinical methods should always be used by the trained observer to supplement and confirm each other.