The Social Security Disability Insurance (DI) program provides benefits to nearly 9 million disabled workers, almost six times the number who received benefits in 1970. There are concerns about future adequacy of the Social Security trust fund, which funds the program. Growth in the DI caseload can be traced to factors including demographics, the labor market, and federal policy. In addition, DI beneficiaries' eligibility for other benefit programs can affect their decision to apply for DI, affecting caseload growth and federal budget outlays. This brief describes these program interactions and what they mean for the DI program.