The objective of this chapter is to introduce the thoracentesis. Thoracentesis or pleural tap is a minimally invasive procedure aimed at extracting fluid from the pleural space. Thoracentesis may have a diagnostic purpose (to understand the cause of pleural effusion) or a therapeutic purpose (to alleviate dyspnea caused by pleural effusions which compress the lung). Thoracentesis is indicated for the diagnostic purposes in all pleural effusions of unknown origin. Thoracentesis is also indicated for therapeutic purposes in massive pleural effusions resulting in breathlessness, respiratory failure, and shift of the mediastinum. This chapter also covers the Causes and mechanisms leading to pleural effusions, patient preparation and position, preprocedure planning, risks and possible complications, post-procedure care and interpretation of pleural fluid analysis. Pleural effusions are commonly distinguished into transudates and exudates on the basis of biochemical characteristics of the fluid, which reflect the physiologic mechanism of its formation.