Final answer:
The Eighth Amendment to the U.S. Constitution prohibits cruel and unusual punishment for those convicted of crimes.
Step-by-step explanation:
The amendment that prohibits "cruel and unusual punishment" of those convicted of crime is the Eighth Amendment to the United States Constitution. This provision is particularly controversial as it has been invoked in debates over forms of capital punishment, including lethal injection and firing squads.
The Supreme Court, while not having a definitive test for what constitutes cruel and unusual punishment, has allowed most penalties short of death for adults, barring instances where the punishment would be deemed disproportionate or excessive.