Shoulder injuries, including acromioclavicular (AC) joint separations, remain a common reason for presentation to the emergency room. Although the diagnosis can be made apparent through proper history and physical examination by the emergency medicine physician, ascertaining the degree of injury can be difficult on the basis of clinical evaluation alone. While there is consensus in the literature that low-grade AC joint injuries can be treated with conservative management, high-grade injuries will generally require surgical intervention. Furthermore, the treatment of grade 3 injuries remains controversial, making it incumbent upon the radiologist to become comfortable with distinguishing this diagnosis from lower or higher grade injuries. Imaging of AC joint injuries after clinical evaluation is generally initiated in the emergency room setting with plain film radiography; however, on occasion, an alternative modality may be presented to the emergency room radiologist for interpretation. As such, it remains important to be familiar with the appearance of AC joint separations on a variety of modalities. Another possible patient presentation in both the emergent and nonemergent setting includes new onset of pain or instability in the postsurgical shoulder. In this scenario, the onus is often placed on the radiologist to determine whether the pain or instability represents the sequelae of reinjury versus a complication of surgery. The purpose of this review is to present an anatomically based discussion of imaging findings associated with AC joint separations as seen on multiple modalities, as well as to describe and elucidate a variety of potential complications which may present to the emergency room radiologist.