In order to appreciate how medical review papers vary, it is important to recall the processing of evidence. Evidence is categorized based on how it has been processed and been packaged for use in clinical practice. They fall into the categories of Non-appraised Research, Critically Appraised Research, and Summaries/ Guidelines. The following content is based largely on the 3rd edition of Users' guide to the medical literature: a manual for evidence-based clinical practice by Guyatt G, et. al, (2015).