This paper reviews the development of the methods for estimating morphometric parameters describing the human lung. The original work was done by light microscopy, but physiologically relevant data depend on studying the delicate structure of alveoli and capillaries by electron microscopy. Methodological improvements of recent years permitted this approach. This yielded estimates of alveolar surface which are by 80% larger than the original data; the reasons are explained by the gain in resolution. On the basis of these data, and by accounting for additional experimental information on the functional availability of gas exchanging surfaces, estimates of pulmonary diffusing capacity (DL) in the range of 90--190 ml O2 . min-1 . mmHg-1 are obtained, which compare reasonably well with physiological estimates of DL obtained under conditions of work.