Myeloid-derived suppressor cells (MDSCs) are currently among the most actively studied group of cells in the tumor microenvironment. They play a key role in protecting the tumor from the immune response and are involved in metastasis and tumor progression. Despite changes in the understanding and classification of what MDSCs are, they are still described as immunosuppressive cells downregulating T cells and arising in chronic inflammation and late-stage cancer. This review collects the latest data on the origin and functioning of MDSCs; examines the main mechanisms of their protumor activity, including their effect on immune response, angiogenesis, and tumor metabolism; and identifies promising areas for further study of MDSCs as a key component of tumor stroma. © 2025 Elsevier B.V., All rights reserved.