With the use of plain radiography, CT, sonography, and more recently MR imaging, radiology has played a pivotal role in the diagnosis and treatment of patients with pleural effusions. The expanding use of CT scanning in patients with pleural disease has enhanced not only our understanding of the factors that affect the distribution of pleural fluid within the thorax, but also our understanding of pathologic and pathophysiologic changes in patients with pleural disease. This paper reviews significant recent developments in this area.