A yellow-dog contract is an unlawful contract that forces employees to agree not to join a union or participate in any union activity as a condition of employment. The National Labor Relations Board (NLRB) is entrusted with administering federal labor laws which prohibit yellow dog contracts.
In the 1920s, corporations forced workers to sign away their rights to unionize as a precondition of employment. These contracts became known as "Yellow Dog Contracts", since employees were deemed to have to cower before their "masters" to get a job. In 1932 Senator George Norris and Congressman Fiorello LaGuardia succeeded in passing the Norris-LaGuardia Act (NLA), which outlawed the practice.