In this chapter, we address general concepts of magnetic resonance imaging (MRI) of bone marrow, with emphasis on normal and common diffuse abnormal marrow patterns that may be difficult to distinguish from the wide range of normal MRI patterns. MRI plays a key role in marrow imaging because of its high sensitivity for detecting focal or diffuse alterations in marrow content. The T1-weighted spin-echo (SE) sequence is the most important sequence for bone marrow MRI. It is superb in demonstrating changes in the amount of fat cells that occur in almost any abnormal marrow condition. To increase specificity, and sometimes sensitivity, it has to be combined with other sequences. Fat suppression plays an important role in bone marrow imaging, as suppression of fat proton signal improves marrow lesion detection. Commonly used are fat suppressed T2-weighted Fast SE (FSE), Dixon techniques (that do not suppress tissue signal but allow multiple types of reconstruction each displaying a certain tissue component, short tau inversion recovery (STIR), and fat-suppressed Gd-chelate-enhanced T1-weighted sequences). Signal intensity of normal lumbar vertebral bodies must be higher than that of adjacent intervertebral disk in an adult patient on T1-weighted images. With aging, the originally red hematopoietic marrow is replaced by yellow marrow, a process that follows a distinct pattern and is reflected by the change of signal intensities. This process is influenced or even reversed (reconversion) by many endo- and exogenic factors that will be discussed. Numerous variations in marrow appearance at MRI must be recognized to avoid confusion with significant marrow lesions.