One of the requirements before you get your AdSense payment is to get your Personal Identification Number or PIN. A PIN is mailed to an AdSense publisher when his earnings reach $50. And it has to be entered to the publisher's account to avoid the payment's getting hold. 
Here are other information regarding the AdSense PIN:
We will mail your PIN to the payment address in your account. You'll then have 180 days to enter your PIN within your publisher account, or to request a replacement. If you do not enter your PIN within this period, your account will be disabled and your earnings refunded to the appropriate advertisers. - Google AdSense Help center
Many concerned publishers ask whether they'll need to complete the PIN process more than once -- such as before receiving every check, or every time they generate $50 in earnings. Fortunately, the answer is no. You'll only need to enter in a PIN once per account, and we'll send it to you the first time you reach $50 in earnings. - Inside AdSense
Also, if you are concerned as to why you were not issued a PIN, one reason could be you signed up for an AdSense account before January 28, 2005. You won't be needing to enter a PIN if that's the case.
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