Magnetization transfer (MT) imaging is a new MR imaging technique in which off-resonance radiofrequency pulses are used to selectively saturate protons in macromolecules. This saturation effect is transferred subsequently (by dipolar and chemical exchange interactions) to protons in free water, thereby altering tissue relaxation times and modulating image contrast. Gadolinium enhancement is not significantly mediated by macromolecular interactions and is, therefore, not suppressed by MT pulses. The theory underlying the use of the MT technique is presented, with examples of its clinical usefulness in improving contrast enhancement for a variety of central nervous system diseases.