Metastases to the pleura are much more common than primary cancers of the pleura. Pleural metastases produce pleural effusions and pleural-based nodules or masses or pleural thickening, features that are also produced by primary tumors of the pleura and by reactive processes. Whereas DMMs account for about 1,500 pleural effusions in the USA each year, metastatic cancers account for about 200,000 pleural effusions each year, a ratio of greater than 130 metastatic cancers to 1 DMM. The most frequent metastatic cancers causing pleural effusion are lung carcinoma (about 60,000), breast carcinoma (about 50,000), and lymphoma (about 40,000). The remaining metastatic tumors to the pleura include virtually every known malignancy, including carcinomas from various sites, sarcomas, and melanoma. Table 2.1 summarizes the incidence of primary sites for malignant pleural effusions.