(TLDR version located at bottom of post) In Simple Terms: So what does this mean? I will leave out the luscious conspiracy theories and stick to the facts. It means that Steam is going to require ANYONE that trades to have their Steam Account protected by Steam Guard for a minimum of 15 days before they are able to trade. All I have to say is: "IT'S ABOUT TIME!" Why? Conspiracy aside, here's what Valve says: TLDR: It is a preventative measure to keep your account from being hijacked or phished. How does Steam Guard work? Once you have verified your email address with Steam, when you login from any computer or device that is not trusted/recognized, a short code will be sent to your email address. Therefore, unless your email address is also compromised by the same person, it is unlikely that they will be able to take control of your Steam account. In fact, Steam Guard will let you know if someone has your password. Since you only receive the email with the short code when someone has successfully entered your Steam username and password on another computer, you will know that you have been compromised since you obviously aren't logging in elsewhere. The Issue: Hasn't Steam Guard been around a while? Yes. Don't people still get hijacked with Steam Guard enabled? Yes. How? A Steam Account's username and password is compromised as well as their email address and password (usually through phishing, or viruses/trojans/keyloggers). The standard explanation for this is that the attacker uses the same password for the email as is used on the Steam account. Here's a good rule of thumb: DO NOT use the same password for Steam and your email account. Typically, phishing websites will ask the user to enter their steam username, password, and email address (and sometimes the email password as well). They try to claim they need the email address to notify you of winning a "prize." More Security, Anyone? Let's add an additional layer of security on top of the additional layer of security! What if I told you that I would almost feel comfortable giving you not only my Steam username and password, but my email address and password, and feel fairly confident that you would not be able to access either? I use this situation "loosely" because I never say never. No, I am not insane. I have a Gmail account. Gmail, like Steam, has a 2-factor authentication method. Google has branded this "2-Step Verification." If you don't have a Gmail account, you should get one simply for Steam. Below you will find information on the benefits of Google's 2-Step Verification. If you take away nothing from the above video, here is the MOST IMPORTANT SECTION: ...helps protect your account even if your password is stolen or cracked. Here's a little more information on Google 2-Step Verification: Google 2-Step Verification: How it Works How You Sign In Using 2-Step Verification Setting Up 2-Step Verification So, now you can have Google send you a text or call you when someone tries accessing your email from an untrusted computer, just like Steam! I strongly recommend this setup to anyone who is wanting protection for their Steam account. TLDR Version: Starting December 12, 2012, Steam Guard has to be enabled for 15 days minimum on your Steam account to be able to trade. Why? Prevents as many successful hijacking/phishings/keyloggers. To enable Steam Guard, verify your email address with Steam. Steam Guard will send an email to your verified with a short authentication token when you login from a new computer. Get a Gmail account. It offers 2-Factor authentication. Sends you a text message or calls you when someone logs in from a different computer.