Lesions of the posterior branch of the axillary nerve are quite common, but a diagnosis is often not made. To make the diagnosis, the finding of diminished sensation over the deltoid muscle in the area supplied by the axillary nerve is key. The anatomy of the axillary nerve, mechanism of injury, and clinical presentation are discussed in this chapter. Wasting of the posterior deltoid and sensory loss over the lateral aspect of the upper arm are shown including photographs.