Pleural effusion is characterized by the pleural cavity filled with fluids, and it is developed by various etiologies.Pleural effusion is not a disease, but the clinical manifestations and signs of the diseases.Therefore, it is not the end that we get the diagnosis of pleural effusion, while it is our ultimate goal to find the etiologies.Both the etiologies and mechanisms of pleural effusion are complicated, so it is very difficult to confirm the ultimate diagnosis. In clinical practice, it would be essential for diagnosis process that includes the analysis of transudative or exudative effusion, the differentiation of benign and malignant effusion, the noninvasive examination in priority, and the invasive examination in necessity.The appropriate application of the integral and dialectical thinking is the art in the diagnostic consideration.