This is a sub book, # 18, of the more extensive UTD Student and Diver Procedures Manual. In this sub-book we discuss the UTD/DIR Principles of Ratio Deco. A a simple "on the fly" strategy of applying a set of rules to develop your "decompression strategy" that will minimize the risks and maximize the benefits of the dive for you and your team. No matter what "your" range, environment or equipment (open circuit, semi closed circuit or full closed circuit), ratio deco will help you approach the decompression as a team, with a consistent approach and strategy and most importantly you and your teammates will understand the risks vs the benefits and how to best deal with the decompression, especially when something goes wrong. Your Ratio Deco Strategy will be derived from a combination of the best of the different decompression theories (past and future), existing software profiles, past and present ideologies and ultimately your experience with the profiles, all combined into one easy to use "on the fly" strategy or methodology. Ratio deco is applicable and consistent within any environment, throughout any range of diving, utilizing any system (open or closed) but most importantly, simple to use by the diver and/or team, especially during the dive; if any changes occur to the planned depth or bottom time the deco can easily be adjusted quickly.