Extension of the fingers is a complex function carried out by simultaneous action of extrinsic and intrinsic muscles, as well as retinacular structures in the dorsum of the wrist, hand, and fingers that support and coordinate the action of the muscles. The extensor mechanism of the fingers is divided into topographic zones, which extend from the forearm to the distal phalanx. Magnetic resonance (MR) imaging shows in detail the musculotendinous and retinacular structures of the extensor apparatus. In the different extensor zones, MR imaging findings are similar to those seen macroscopically in anatomic sections. Understanding of and familiarity with the extensor anatomy of the hand and fingers by the radiologist is crucial for better assessment of pathologic conditions with MR imaging and optimization of this modality as a diagnostic tool. Extensor tendon injuries and tenosynovitis represent clinical situations in which knowledge of this anatomy is useful for the clinical radiologist.