Due to the high number of accounts getting hacked, you are strongly encouraged to get a Blizzard Authenticator to protect your account. Once the authenticator is tied to your account, you get a special pet to show everyone that you take security seriously. See this forum thread for details:

http://forums.worldofwarcraft.com/thread.html?topicId=22048397570&sid=1

You can purchase your authenticator at the Blizzard Store for only $6.50. (Don't buy it from the reseller at Amazon who's reselling them for $50!)

http://us.blizzard.com/store/details.xml?id=1100000822

If you have an iPhone or an Android phone (such as the Motorola Droid), you don't even need to purchase an authenticator. Just download a free authenticator phone application and use your phone to log in to WoW. Details can be found at the Battle.net site, here:

https://us.battle.net/account/support/mobile-auth-download.xml

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