Malignant pleural and pericardial effusions are a common problem in the treatment of patients with lung cancer, breast cancer, or lymphoma and may occur with any malignancy. These effusions are frequently symptomatic and, in the case of the pleural space, may be the presenting sign of cancer. In other patients, they represent markers of recurrent, disseminated, or advanced disease. Given the poor prognosis of most patients presenting with these effusions, reducing symptoms and improving quality of life are the primary goals of treatment. Permanent drainage and/or obliteration of the pleural or pericardial space are crucial to the effective management of the effusion and will provide long-term palliation. Immediate relief can be accomplished via external drainage, but definitive therapy may often also require interventional radiology, cardiology, and thoracic surgery, as well as medical and radiation oncology. The pathophysiology, diagnosis, and treatment of malignant pleural and pericardial effusions are discussed in this article.