The Federal Employees' Retirement System Act of 1986 (codified as amended largely at 5 U.S.C. 8351 and 8401 through 8479) originally limited TSP eligibility to specifically named groups of employees. On various occasions, Congress has since expanded TSP eligibility to other groups. Depending on the circumstances, that subsequent legislation requires retroactive contributions or provides other special features. Where necessary, this part describes those special features. The employees and employing agencies covered by this part are also governed by the other regulations in 5 CFR chapter VI to the extent that they do not conflict with the regulations of this part.

The definitions generally applicable to the Thrift Savings Plan are set forth at 5 CFR 1690.1.

The employing agency is responsible for transmitting to the TSP record keeper, in accordance with the TSP record keeper's procedures, any employee and employer contributions that are required by this part.

An employing agency must notify affected employees of the application of this part as soon as practicable.