If you work while you are collecting disability benefits, special rules apply:

Trial work period – you can work up to 9 months in any five year period and continue to collect disability benefits.  There is no cap on your earnings during the 9 month trial work period.  If you earn more than $670 in a month in 2008, that month counts as a trial work period month.  In 2007, the trial work period trigger was $640, and in 2006, the trial work period trigger was $620.

Extended Period of Eligibility. After you have used up your nine months of trial work , Social Security looks at your earnings on a month by month basis for a 36 month "extended period of eligibility."  During the extended period of eligibility, you receive your regular SSDI check if your earnings are not "substantial."   If your earnings are "substantial" you are not eligible for a check in that month.  How much is "substantial?"

2009- $980 per month
2008-  $940 per month
2007 – $900 per month
2006 – $860 per month

If you earn more than the above limits for a particular month, you will be asked to repay Social Security for any disability benefits received during that month.  SSA will literally review your earnings month by month – if you exceed the limit for a particular month, you will have to pay it back.

In addition, if your earnings record shows a pattern of work, SSA will likely review your case to determine whether benefits should be stopped.  This is called a "continuing disability review" and is an eligibility issue rather than a money issue, although the two often go hand in hand.

Note that special rules apply for blind disability claimants – a blind person can earn more money before his earnings are deemed "substantial."