The pleural space is a virtual space between the lung and chest wall. One may therefore intuitively predict that the characteristics of these two structures will also affect the characteristics of the pleural space. Thus the factors holding the lung against the chest wall will be described briefly, followed by a discussion of the effects of respiratory muscle contraction and thoracic shape on pleural pressure. In this section we shall refer mainly to animal experiments , from which most of the direct evidence is derived. Subsequently, we shall examine the static, quasistatic, and dynamic distribution of gas in the lung and how this distribution is affected by the thorax. There we will refer mainly to human experiments since most of the studies could be performed in humans and because of substantial interspecies differences. Because we believe that in the upright and horizontal posture, the basis for the gradient in alveolar expansion and gas distribution differs, we shall use this division in describing and interpreting the results from the literature. Finally, the effects of the lung on the chest wall will be discussed, and practical questions for further research will be briefly outlined.