The diagnosis of a soft tissue mass in children is a common clinical situation. Most of the lesions are benign and can be treated conservatively or by non-mutilating surgery. Nevertheless, the possibility of a malignant soft tissue tumor must be systematically considered. The most frequent benign soft tissue lesions in children are vascular lesions, fibrous and fibrohistiocytic tumors and pseudotumors, whereas rhabdomyosarcomas account for 50% of all soft tissue sarcomas. A child presenting an atypical soft tissue mass should be managed by a multidisciplinary centre, and primary resection must be proscribed until a definite diagnosis has been established. The role of imaging is essential either to confirm the benign nature of the mass or to give arguments to perform a diagnostic biopsy. Clinical examination, conventional radiography and ultrasound with Doppler represent the first-line examinations and are sometimes sufficient to assess a diagnosis. In all other situations, MRI is mandatory to establish the probable nature of the lesion and to assess local extension.