A bone marrow edema pattern on MRI has a similar signal quality as an inflamed synovium and may, in fact, reflect true inflammatory infiltrates rather than a pure accumulation of extracellular fluid. Bone lesions near sites of rheumatoid arthritis-related inflammation are heavily vascularized, contributing to the high water content and enhanced visibility on MRI. However, even without erosive change, periarticular bone marrow lesions may be seen. This chapter describes the nature of bone marrow lesions detected by MRI in patients with inflammatory arthritis.