With the increasing use of intensity-modulated radiation therapy (IMRT) for the treatment of head and neck cancer, radiation oncologists are expected to have an in-depth knowledge of the computed tomographic (CT) and magnetic resonance (MR) imaging anatomy of this region to be able to accurately characterize tumor extent and define organs at risk for potential radiation injury. The brachial plexus is a complex anatomic structure in the head and neck adjacent to diseased nodes and elective nodal volumes (ie, nodal areas that are prophylactically treated because they are at high risk for micrometastatic disease) and should, therefore, be carefully identified and contoured at CT prior to IMRT planning. A number of multi-institutional protocols mandate contouring the brachial plexus as an "avoidance structure" (ie, a structure or volume that is at risk for complications of radiation therapy) in the planning of head and neck radiation therapy, and, although little information exists on the best method of doing so consistently, contouring may be facilitated with fusion CT-MR imaging software. With three-dimensional conformal radiation therapy, the brachial plexus is not routinely contoured; therefore, its dose limits are not evaluated in treatment planning. In contrast, with IMRT, tolerance doses can be set to limit the maximum dose to the brachial plexus to 60 Gy in most radiation protocols, although the true radiation tolerance dose in patients with head and neck cancer has been mentioned only sporadically in the literature. Additional studies will be required to determine if identification of the brachial plexus as an avoidance structure prior to radiation therapy planning improves treatment outcome in patients with head and neck cancer and reduces long-term toxicity in this structure.