Desmoid tumors, or aggressive fibromatosis, are rare, non-metastasizing soft tissue neoplasms characterized by local invasiveness and a high risk of recurrence. This chapter provides a comprehensive and radiation oncology–focused review of desmoid tumors, encompassing their molecular biology, clinical presentation, diagnostic strategies, and evolving management. Central to tumorigenesis are alterations in the Wnt/β-catenin pathway—particularly CTNNB1 and APC mutations—which inform both prognosis and therapeutic responsiveness. While active surveillance and systemic therapies remain integral for select cases, radiation therapy has emerged as a pivotal modality, particularly for unresectable, recurrent, or anatomically complex tumors. Advances in precision radiation—such as IMRT, IGRT, MR-guided adaptive RT, and proton therapy—now allow for highly conformal dose delivery with excellent local control and reduced toxicity. The chapter underscores the importance of radiotherapy not only as an alternative to surgery in functionally critical regions but also as an adjuvant tool in margin-positive resections. It also explores future directions, including radiogenomics, molecular imaging, and combined modality approaches that may further enhance the therapeutic ratio. Designed for radiation oncologists and trainees, this resource highlights how state-of-the-art radiation strategies are transforming the management of this once surgically dominated disease.