This app is likely to be overlooked and under-rated, but I believe it is a hidden gem. Someone (no developer or contact information is provided) has compiled an incredibly comprehensive list of injury and disease classification systems about the shoulder with their associated primary literature reference. The app is organized into list format, and the main list tree includes conditions such as AC joint, Acromion, SLAP, Calcific Tendonitis, etc. Then, upon selecting a particular condition or injury, you see multiple different classification systems that have been published for that particular topic. This list is very comprehensive and utilizes the most common classification systems, as well as those less frequently employed. Each description includes a reference to the primary literature source, a brief description, and typically includes an associated image when appropriate. Overall, the developers of this small, simple, and free app have done a phenomenal job of compiling classification systems for shoulder pathology.