A revocation hearing may be requested by a probation officer who believes that an individual has violated the terms of their probation. Revocation hearings are typically conducted by the original sentencing court. If the court determines that the individual violated their probation, they may revoke the probation and re-instate the prison sentence.
It is unclear whether a right to counsel exists at the probation revocation hearing although at least one court found that the right existed when sentencing and a probation revocation hearing were held at the same time. 
- Mempa v. Rhay, 389 U.S. 128 (1967)