Simple SummaryThe diagnosis of retroperitoneal sarcoma may be challenging for the radiologist. Current guidelines report postsurgical margin as the strongest predictive factor for disease-specific survival and recurrence, as well as histologic subtype and grade. The role of the radiologist is indeed important in RPS diagnosis, management, and follow-up, as the ability to promptly recognize local progression, invasion of nearby structures, and complications has a direct impact on patients’ survival. A practical guide is provided to radiologists with an overview of the current knowledge regarding cross-sectional CT/MRI imaging features of patients with retroperitoneal sarcomas, presenting tips and tricks to improve imaging diagnosis of RPS.AbstractPrimary retroperitoneal sarcomas (RPS) represent around 10–16% of all sarcomas, with liposarcomas and leiomyosarcomas being the most common subtypes. RPS have some peculiar characteristics, imaging appearances, worse prognosis, and complications compared to other locations of sarcoma. Commonly, RPS primarily present as large masses, progressively encasing adjacent structures, causing mass effect, and complications. RPS diagnosis is often challenging, and these tumors may be overlooked; however, failure to recognize RPS characteristics leads to a worse prognosis for the patients. Surgery is the only recognized curative treatment, but the anatomical constraints of the retroperitoneum limit the ability to achieve wide resection margins; therefore, these tumors have a high rate of recurrence, and require long-term follow-up. The radiologist has an important role in the diagnosis of RPS, the definition of their extent, and their follow-up. Specific knowledge of the main imaging findings is required to reach an early diagnosis, and, ultimately, to guarantee the best patient management. This article provides an overview of the current knowledge regarding cross-sectional imaging features of patients with retroperitoneal sarcomas, presenting tips and tricks to improve imaging diagnosis of RPS.