In the last two decades, subacromial impingement syndrome has become an increasingly common diagnosis for patients who have a painful shoulder. However, subacromial impingement syndrome is a specific diagnosis and is not the only cause of pain in the anterosuperior aspect of the shoulder. Impingement may be difficult to diagnose because the clinical presentation may be confusing. It is important to differentiate subacromial impingement syndrome from other conditions that may cause symptoms in the shoulder, such as glenohumeral instability, cervical radiculitis, calcific tendinitis, adhesive capsulitis, degenerative joint disease, isolated acromioclavicular osteoarthrosis, and nerve compression. This is particularly true when examining younger patients, especially athletes who perform overhead motions with use of the upper extremity, in whom the diagnosis of impingement should be made with caution. In many cases, the primary diagnosis is subtle glenohumeral instability even though impingement and subacromial bursitis are evident.