Primary retroperitoneal masses are a rare but diverse group of benign and malignant processes. Magnetic resonance (MR) imaging is playing an increasing role in evaluating retroperitoneal soft-tissue masses. Since the MR imaging features of most retroperitoneal soft-tissue masses are nonspecific, prediction of a specific histologic diagnosis remains a challenge for the radiologist. However, there are certain specific MR imaging appearances that are helpful. Dynamic enhancement patterns can reflect the vascularity of masses, differentiating benign from malignant soft-tissue masses. This article pictorially illustrates the MR imaging features of various common and uncommon retroperitoneal masses.