A wide spectrum of entities may give rise to soft-tissue masses in children, including benign and malignant tumors, pseudotumors, and both neoplastic and nonneoplastic vascular lesions. Because of its excellent tissue contrast, multiplanar capability, and lack of ionizing radiation, magnetic resonance (MR) imaging has become the modality of choice in the evaluation of deep and large soft-tissue masses in children. In the vast majority of cases, however, accurate interpretation of the MR imaging findings requires correlation with the clinical findings. For example, in most posttraumatic and inflammatory pseudotumors, the clinical history is fundamental to establishing the diagnosis. In the evaluation of periarticular cysts, the location of the mass and its relationship to a joint are crucial for diagnosis, whereas in the evaluation of vascular lesions, including hemangiomas and vascular malformations, clinical findings combined with MR imaging findings are needed for accurate diagnosis in most cases. The identification of fat within adipocytic tumors is useful, but tissue biopsy may be required for final diagnosis. Nevertheless, MR imaging is useful in determining the origin and character of pediatric soft-tissue masses, defining their extent and their relationship to adjacent structures, and performing posttherapy follow-up.