Definitive determination of the cause of articular swelling may be difficult based on just the clinical symptoms, physical examinations and laboratory tests. Joint disorders fall under the realms of rheumatology and general orthopaedics; however, patients with joint conditions manifesting primarily as intra-articular and peri-articular soft tissue swelling may at times be referred to an orthopaedic oncology department with suspicion of a tumour. In such a situation, an onco-radiologist needs to think beyond the usual neoplastic lesions and consider the diagnoses of various non-neoplastic arthritic conditions that may be clinically masquerading as masses. Differential diagnoses of articular lesions include infectious and non-infectious synovial proliferative processes, degenerative lesions, deposition diseases, vascular malformations, benign and malignant neoplasms and additional miscellaneous conditions. Many of these diseases have specific imaging findings. Knowledge of these radiological characteristics in an appropriate clinical context will allow for a more confident diagnosis.