PADI (The Professional Association of Diving Instructors) is by far the biggest and most recognized scuba diving certification agency in the world. It's usually possible to find a PADI instructor in any diving destination and more beginning divers are certified by PADI than by all of the other scuba diving agencies combined.
PADI is very active in marine conservation through their Project AWARE operation, as well as being the biggest advocate of diving through advertising and sponsorship. Not to mention, PADI is involved in technical scuba diving training, and also Emergency First Response, an organization providing CPR and First Aid Training.
Other well-known scuba diving certification agencies:
-SSI (Scuba Schools International)
-YMCA (Young Men's Christian Association)
-BSAC (British Sub-Aqua Club)
-NAUI (National Association of Underwater Instructors)
