Among all neoplasms of the mediastinum, those composed of mesenchymal elements, and arising primarily in mediastinal soft tissue, are the least common group of tumors discussed in this issue of Seminars. Apart from tumors of nerve sheath, neuroectoderm, adipose tissue, and lymphatic vessels, few of them will comprise a significant part of the surgical pathologist's practice. Yet each poses two important clinical problems: the recognition of visceral-associated (as opposed to primary soft tissue) lesions, and the exclusion of metastases from an extrathoracic site. In this review, the histologic, immunohistochemical, and ultrastructural features that characterize mediastinal soft tissue tumors will be emphasized. Clinical aspects of these lesions are also discussed, particularly as they may relate to the aforementioned clinical questions.