For Social Security disability, retroactive disability benefits can be paid for up to 12 months prior to the date you filed your claim, if you were disabled during that time. 

Here is a simple example:

Assume that a person filed for Social Security disability benefits today, August 1, 2009.  Disability benefits are available for the past 12 months, depending upon the claimant's onset date. If the claimant became disabled on June 1, 2008, then the retroactive benefits would begin after the 5 month waiting period, on November 1, 2008.

However, if the onset date were earlier, say on March 1, 2008, then the full 12 months of retroactive benefits would be paid, since the 5 month waiting period would be over by August 1, 2008. Benefits would begin on that date. An onset date 17 or more months prior to your filing date maximizes your retroactive disability benefits.

For an SSI claim, benefits simply begin on your filing date if your claim is granted. There is no waiting period for elegibility, and there are no benefits available prior to your filing date for SSI.