There are a number of agencies which certify divers. They work by training and certifying instructors in their syllabus and teaching methods, and then allowing those instructors to certify individual divers.

All reputable dive shops will require certification of your skills in the form of a certification card (C-card) from a recognized agency.

PADI

The Professional Association of Diving Instructors is the largest scuba certification agency, issuing about half a million new certifications a year. PADI is a commercial agency targeted towards recreational divers who want to learn quickly. A basic PADI Open Water course can be completed in as little as three days, although it is generally advisable both to allow more time and to take the more thorough Advanced Open Water course.

Their pre-open water program has two courses: Discover Scuba, a taster course; and PADI Scuba Diver, allowing you to dive with an instructor. Their open water certification called "PADI Open Water Diver." After you've completed Open Water, a number of Adventure Dives and Specialty courses in interests like underwater photography and fish identification and skills like enriched air open up. Continuing recreational skill level is reflected in the Advanced Open Water course, allowing you to dive to 30 meters; the Advanced skills of deep diving and wreck diving; and the Rescue Diver and Master Scuba Diver certifications.

NAUI

The National Association of Underwater Instructors is US-based and is the oldest recreational scuba certification agency. Their pre-open water program is the NAUI Skin Diving course in snorkelling and breathe hold, there's no pre-open water scuba certification. Their open water certification is called "NAUI Scuba Diver". After you've achieved Scuba Diver level, you can proceed to Advanced Scuba Diver, a number of interest and skill courses, and the Master Scuba Diver certification.

GUE

Global Underwater Explorers diver training is geared towards cultivating a diving style that is functional across several environments and that assists divers to have more fun with less stress and greater safety. Distributed across a diverse educational curricula#cave diving, technical diving, recreational diving, and rebreather diving.

CMAS

The French-based Confédération Mondiale des Activités Subaquatiques is a volunteer-run amateur organization that takes a more comprehensive approach than many of the above. CMAS's basic "one-star" certification usually involves training several days a week for several months, including physics and physiology lessons and practicing a wide variety of skills, and is considered roughly equivalent to PADI Advanced Open Water. Local CMAS chapters can be found in most countries around the world, particularly Europe, and it's common (and recommended) to complete a CMAS certification in your home country before that big trip to tropical reefs.

IANTD

The International Association of Nitrox and Technical Divers specialises in training programs in more advanced recreational diving including: wreck diving; cavern diving; rebreather diving and diving with different gas mixtures including helium mixtures. Many of the skills taught are outside the scope of even the advanced courses offered by PADI and similar organisations.

PDIC

The Professional Diving Instructors Corporation

IDEA

The International Diving Educators Association

ACUC

The American Canadian Underwater Certifications

BSAC

The British Sub Aqua Club

SSI

Scuba Schools International