Clinico-pathological correlation has always been one of the main aims of the morbid anatomist. However, although clinical physiologists have for many years expressed their results in a quantitative form, pathologists have progressed mainly, though not entirely, along a descriptive path. Descriptive pathology has reached a high degree of refinement with the study of ultra-structure by the electron microscope and the probing of cellular chemistry by histochemical methods. Little interest has been shown in the field of quantitative morphology, although a quantitative study of gross and microscopic pathology, correlated with the findings of physiopathologists, might well yield much useful information. Thompson (1917) pointed out many applications of this approach to general biology, and recently Grant (1961) has drawn attention to the importance of this type of study in cardiac pathology and has referred to the work of Linzbach (1960). This paper is concerned with the methods of quantitation as applied to the lung, but the methods mentioned are capable of adaptation to almost any organ. Pulmonary physiology is a subject which has undergone enormous advances over the past 30 years, mainly since the introduction of cardiac catheterization as a safe clinical procedure by Cournand and Ranges (1941). The relatively slow progress of pulmonary pathology is probably due to inadequate descriptions of the variousaspects of emphysema and chronic lung disease. This has been remedied largelyby the work on chronic bronchitis of Reid (1954) and the description of centrilobular emphysema by Leopold and Gough (1957). What is needed now is precise information concerning volumes of normal and abnormal regions of the lung, the surface area of the airtissue interface, and the number of the various units, such as alveoli and alveolar ducts, in normal and emphysematous lungs. Thepurpose of this paper is to show how these measurements can be