Transudates, most usually, have a systemic cause, whereas the cause for exudates is most often regional. Bilateral pleural effusions are more frequently transudates than exudates. The reverse is true of unilateral pleural effusions. Thus, the approach to unilateral and bilateral pleural effusions conceptually differs. This chapter outlines the diagnostic approach to the bilateral pleural effusions. It lists the etiologic possibilities, describing the pleural fluid characteristics in each case. It also discusses unusual situations wherein bilateral pleural effusions are exudative rather than transudative, and examines the conditions responsible for the same.