The pathological diagnosis and classification of malignant sarcoma: “sarcoma” is the common name for malignant tumors of mesenchymal origin. The Greek word “sar” means fish-like, as most of these tumors have a fish-like appearance. Compared to the common epithelial cancers such as lung and liver cancer, malignant sarcomas are rare, but they have an unusually wide variety of histologies. Most malignant sarcomas occur in soft tissues and bones containing muscle, fat, fibrous tissue, blood vessels, etc. in the extremities or retroperitoneum; occasionally, they also occur in internal organs, with smooth muscle sarcomas in the uterus or gastrointestinal tract being the most common. The pathologic diagnosis of malignant sarcoma is quite difficult. Although well differentiated sarcomas can be diagnosed by conventional staining methods, poorly differentiated sarcomas often require immunostaining, electron microscopy, chromosomal or molecular biology tests. In extremely difficult cases, histological classification is sometimes not possible with the help of these methods. The pathological diagnosis of sarcoma requires clinical data, such as the age of the patient, the location and size of the tumor, and the duration of the disease. The types of sarcoma that occur in children and adults vary considerably; for example, liposarcoma is more common in adults but less common in children, and the opposite is true for embryonal rhabdomyosarcoma. In general, sarcomas occur more often in deeper tissues; only a few exceptions are found in skin or subcutaneous tissues, such as aplastic dermatofibrosarcoma, epithelioid sarcoma, and angiosarcoma. The degree of differentiation of sarcoma tissues is important for prognosis, and therefore tissue grading is specifically included in the staging of sarcoma. It is not easy to have a system of tissue grading that is applicable to all sarcomas. There are also several systems of tissue grading for sarcomas, some using four grades, some using three grades, and some distinguishing only between low malignancy tissue grading and high malignancy tissue grading. The same system can be used by different pathologists, and the number of grades defined cannot be identical. The histologic grading of sarcoma is generally based on the number of cell division phases, cellular structure, cell differentiation, necrosis, and the amount of mucus-like tissue. The histologic pattern of sarcoma is closely related to the prognosis; therefore, newer histologic grading systems incorporate histologic classification. In addition, the size, depth, and location of the tumor often affect the prognosis. Small and superficial tumors usually have a better prognosis. Sarcomas located in the retroperitoneal cavity often have a poor prognosis, even if they are of low malignant histologic classification.