The primary difference between SSI and SSDI is that SSDI applies to those individuals who have an employment history and have therefore paid Social Security taxes for the requisite period of time before their injuries. SSI, on the other hand, is generally petitioned for by individuals who do not have an employment history and have therefore not contributed to the Social Security system for the requisite period of time before being injured (i.e., this may be the case for an individual who has a chronic illness at an early age in life).

The Social Security Administration has a stringent process in place for petitioners seeking SSI or SSDI. First, the individual must not be able to engage in Substantial Gainful Activity as a result of the illness, and the illness must exist for a statutorily determined period of time. Significant medical documentation is required by the petitioner in his or her claim for SSI or SSDI benefits.

Most cases are scheduled for a hearing before an Administrative Law Judge at the Social Security Administration at which the Administrative Law Judge determines the merits of the case. The Administrative Law Judge often relies on his own experts (usually doctors) in determining whether benefits ought to be paid to the petitioner.

Therefore, in this area, failing to prepare is preparing to fail, and enlisting the help of an attorney is crucial to prepare the best possible case. These cases are handled on a contingency fee, which is statutorily determined.