The etiology, diagnosis, investigation and treatment of pleural effusions have been discussed. The frequency of tuberculous origin of socalled “idiopathic pleural effusion” has been reemphasized. Congestive failure, inflammatory processes, and neoplasm are responsible for most of the effusions seen among the general population. The routine procedures for diagnostic laboratory examination of pleural fluid have been evaluated in the light of present knowledge, with the following general conclusions. A. General appearance of the fluid identifies empyema, chylothorax, hemorrhagic effusion and traumatic bleeding. B. Transudate-exudate determinations correlate only generally with the type of process causing the effusion. C. Clotting of the fluid has little effect on protein content and no effect on specific gravity. D. Hemorrhagic effusion is due to malignant neoplasm in over 50 per cent of the cases, and tuberculous effusion is not commonly hemorrhagic. E. Differential counts of the pleural fluid are of most value in differentiating acute pneumonias from tuberculous processes. Polymorphonuclear leukocytes are the predominant cell type in most pneumonias while lymphocytes usually, but not always, predominate in the tuberculous effusions. F. The tubercle bacillus is often demonstrated by guinea pig inoculation and/or culture, and one or both of these procedures should be part of the routine examination of pleural fluids. G. Tumor cells are found in about half of the cases of neoplasm, the percentage being somewhat higher in carcinoma of the breast than with bronchogenic carcinoma. False positives occur. Techniques for diagnostic biopsy of the pleura as well as therapeutic injection of radioactive gold and nitrogen mustard into the pleural space have been discussed. Examination of the pleural fluid is most valuable in making the definitive diagnosis of tuberculosis by culture and of neoplasm by cell block and Papanicolaou smear. The other procedures involved in determining pleural fluid characteristics have such a wide range of values in any given pathologic condition that diagnosis is, at best, presumptive.