Bone marrow edema is easily identified on MRI. The terminology suggests that the water content of bone marrow is increased when it is T1W hypointense and T2W hyperintense. It is a misnomer since, histologically, the abnormality does not correspond to marrow edema. The histological findings vary based on the underlying etiology and the presence of fibrosis or inflammatory infiltrate is frequent and often predominant. In France, this terminology is used routinely to describe such lesions. The term osteitis is preferred to describe these marrow signal changes in the setting of rheumatic joint diseases. The detection of bone marrow edema is important because of its diagnostic and prognostic value. It occurs in isolation and is reversible in patients with bone contusion and complex regional pain syndrome. It indicates underlying structural damage and may modify management in patients with rheumatoid arthritis or spondylarthropathies. MR scores for disease activity rely mainly on the presence of marrow edema. Finally, diffusion weighted MR allows quantification of marrow edema and could be more sensitive than conventional MRI to detect inflammation. The purpose of this article is to review the imaging features of marrow edema, review the underlying etiologies and its diagnostic and prognostic value.