This chapter will cover the pathology of endometrial stromal tumors, including endometrial stromal nodules, low-grade endometrial stromal sarcomas, and high-grade endometrial stromal sarcomas. There has been significant gains in our understanding of the histologic and molecular classification of endometrial stromal sarcomas over the past decade with important diagnostic and prognostic implications. The salient histopathologic, immunophenotypic, as well as molecular findings will be discussed with an emphasis on features that can help to separate these tumor types from each other and from other nonendometrial stromal tumor histologic mimics. Areas of evolving understanding that require further research will also be highlighted.