Of all diving associations, the best known is PADI which operates world-wide and offers several extremely good diving courses. When you're on your scuba-diving travels, the PADI sign displayed at a training school or facility is usually a guarantee of excellence. You can be sure that any training undertaken under the aegis of PADI will be well formulated and well regulated. SSI is also an extremely good association, but not as well-represented throughout the world as PADI (definitely the Betamax to PADI's VHS!), and if you are training to become an instructor, it's possible that it would be easier to get a job with the PADI initials behind you. BSAC is specific to Britain and specializes in cold-water diving. TDI and SDI are sister organizations based in the US, with TDI having a reputation for excellence in courses for technical diving. IANTD is also primarily concerned with technical diving particularly gas mixtures.