The foundation of U.S. criminal procedure is the U.S. Constitution, including the first 10 Amendments, which form the Bill of Rights. The Constitution guarantees all persons living in the U.S. fundamental rights, freedoms and liberties. Chief among these, as far as U.S. criminal law is concerned, is that defendants are entitled to a presumption of innocence. Defendants do not have to prove their innocence. The government must prove their guilt beyond a reasonable doubt. Rights such as these frame the federal-state system prescribed in the Constitution. Of particular importance are the Fifth, Sixth and Eighth Amendments.

The Fifth Amendment protects defendants against double jeopardy (being tried more than once for the same crime by the same authority), and against being required to testify against themselves in criminal cases. Most significantly, it also protects defendants' rights of "due process," a phrase of vast significance in the Bill of Rights that, especially in the 20th century, was interpreted by the courts to confer on defendants a broad array of protections and rights.

The Sixth Amendment guarantees defendants a "speedy and public trial by an impartial jury of the state and district wherein the crime shall have been committed." It also entitles defendants to be confronted by (and to cross examine) the witnesses against them and to have the "assistance of counsel" for their defense. This last protection also has been expanded over the years to, in effect, guarantee all defendants adequate counsel in criminal trials.

The Eighth Amendment rules out "excessive bail" for defendants and prohibits "cruel and unusual punishments." This last prohibition has been interpreted by the courts to limit the kinds of punishments that can be inflicted. In 1972, the death penalty statutes of 38 states were effectively voided based on this constitutional provision. Some were rewritten to pass constitutional muster. Currently, 38 states have a death penalty statute. But the example serves to illustrate that it is the U.S. Constitution that is supreme in the U.S. system, not U.S. criminal law per se. Neither Congress nor the states can pass laws that violate the Constitution.

Every state and the federal government has its own "substantive criminal law" (specifying crimes and defenses) and "criminal procedure" (specifying the stages of the criminal process from arrest through prosecution, sentencing, appeal and release from prison). Each state legislature promulgates that state's criminal law, which is enforced by state and county prosecutors, adjudicated in local and state-level courts, and punished in state prisons or local jails. Congress passes federal criminal laws, which are enforced, prosecuted, adjudicated and punished by federal law enforcement agencies, prosecutors, courts, prisons and probation and parole systems.