These include different names for many characters and monsters, changes to the appearance of the cards to differentiate them from their real-life counterparts and various cuts and edits pertaining to violence, death and religious references to make the series suitable for children. The adaptation received many changes from the Japanese version to tailor it for international audiences. In 2001, 4Kids Entertainment obtained the merchandising and television rights to the series from Konami, producing an English language version which aired in North America on Kids' WB! between Septemand June 10, 2006, also releasing in various countries outside of Japan. An interview with Brian Zimmerman mentions that they were only contracted to do three seasons of the series, meaning it was possibly 144 episodes. It's not known exactly how many episodes were dubbed, though it is known to go as high as 80 episodes. The dub featured an extremely limited cast of only six known voice actors. Recorded at Voiceovers Unlimited and Yuan Production Holdings in Singapore, this dub was produced for Odex Productions. Note also that a short film (titled just "Yu-Gi-Oh!") was produced, which you might want to watch after you've finished the "zeroth" series.An English dub of the series was originally dubbed in Singapore for the South East Asian market. The "zeroth" series is 27 episodes in length, and was never dubbed, so you'll have to hunt down subtitles. As such, you can watch it at any point, though I would recommend watching it just before or after you watch the first series (just to see the contrasting ways in which the two series evolved). I didn't mention the "zeroth" series (the one released in 1998) anywhere in there because it's sort of orthogonal to the rest of the Yu-Gi-Oh! anime franchise. Konami has licensed it for dubbing, but the dubs presumably won't start until the dubs for ZEXAL series 2 are complete.
As of this writing (), it looks like the first 40 episodes have been dubbed, with more coming out at a rate of one per week. Since this series only just ended in Japan (Mar 2014), dubbing is still in progress.
Note that you'll have to hunt for subtitled episodes if you want to watch the fourth (and final) season of GX - it was never dubbed.
Also since this comment was written another Yu-Gi-Oh! movie Dark Side of Dimensions has been released which takes place 6 months after the end of Duel Monsters. It is best to watch it after episode 198 of Duel Monsters as it takes places between 198 and 199. This mini-series was created exclusively for US release (so I guess it's "non-canon" if you care about that), but it's pretty enjoyable nonetheless.