Yes. You have to report your income very diligently. For each $2 you make, they take off $1.
Suppose you had $500 in SSI payments, you made $1000 in a month, you won't get any $ next month from SSI.
SSDI is a different story, you can make up to $1000/mo as of jan 2010 without any deductions. But don't forget there is the trail period, which is any 9 months in a 5 years term where you can make any amount without worrying about your SSDI. They count the trail period months off if you make more than $500 or so.
don't forget that you can keep all of the ssd and work and make less then 720 a month so you can make up to 166.00 a week and not lose the ssd check you can work and make 8 an hr for 20 hrs a week for 160 a week