This text is designed to provide a comprehensive background on rebreathers -- what they do, how they operate, and what is involved in learning to dive them. The chapters cover types and history of rebreathers, theory as it pertains to different designs of units, pre-dive, dive and post-dive, and maintenance procedures for rebreathers, and emergency procedures used with rebreathers. It is not meant to stand alone as a user training program, but may be incorporated into a training program under the guidance of a certified rebreather instructor. We hope you find the information interesting. In many cases, it is unique among standard resources on the topic.