© 2003 by Taylor & Francis Group, LLC. The pleural space is a unique compartment of the body because inflammation is usually associated with the accumulation of fluid and cells in a well-defined, enclosed compartment in the chest cavity (1,2). This allows for a novel window into the inflammatory process, since pleural pathology can be viewed from the moment of initiation of injury to the point of resolution via thoracentesis and/or sampling of pleural tissue (3,3). Until recently, the pleura was considered to be a relatively inactive monolayer of cells whose main function was to serve as a membrane that allowed for smooth movement of the underlying lung (5). Over the past several years, the pleura has been recognized as an inflammatory organ capable of multiple functions (6,7). The pleural mesothelial cell has emerged as the central cell that is capable of initiating, maintaining, and regulating the inflammatory milieu of the pleual space in pleural disorders. The pleural mesothelial cell has a defined apical surface with microvilli and a basilar surface that is in intimate contact with the extracellular matrix. During homeostasis the pleural space is quiescent, with minimal pleural fluid (3-7 mL) lubricating the visceral and parietal surfaces of the pleura and a small number of pleural macrophages and monocytes (7,8).