Bone marrow edema (BME) is a descriptive term which identifies a specific magnetic resonance imaging (MRI) pattern that can be observed in a number of clinical entities, which are often characterized by pain as their main symptom, but show significant differences in terms of histopathological findings, causal mechanisms and prognosis. Bone marrow lesions in the subchondral bone of subjects with knee osteoarthritis (OA) seem to be associated with pain and progression of cartilage damage over time. Some histopathological studies of advanced OA have shown a prevalent fibrosis and bone marrow necrosis. BME of the subchondral bone in rheumatoid arthritis is associated with an infiltrate of inflammatory cells and osteoclasts and has a predictive value of further development of erosions. In spondyloarthritis, BME of the sacroiliac joints identifies an active sacroiliitis and is associated with histological inflammation and radiographic progression, whereas the relationship between BME lesions of the spine and syndesmophyte development is still controversial. BME syndromes (BMES), such as transient osteoporosis of the hip, regional migratory osteoporosis, and transient post-traumatic BMES, are characterized by a BME pattern on MRI and a self-limiting course. The potential evolution of BMES toward osteonecrosis is still controversial.