The U.S. Constitution provides for due process protections in all criminal investigations, beginning at the point a crime is discovered. The framers of the Constitution intentionally stacked the deck against the government in this process in order to make certain that innocent people are not wrongly convicted, as well as balance out the disparity in resources between the state and federal government, and the average accused person.

The foundation of these protections is the presumption of innocence, until guilt has been proven beyond a reasonable doubt. This burden of proof is often misunderstood, and can be difficult to quantify or explain. The reasonable doubt standard does not mean that a juror must be 100% certain of guilt, as this would mean that almost every criminal would go free at trial. However it also must mean something more than “more sure than not”. The Texas judiciary has defined reasonable doubt as "the kind of doubt that would make a reasonable person hesitate to act in the most important of his affairs.”

In addition to Due Process, the Constitution also provides that “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.” In making these guarantees, the Fourth Amendment raises issues about what is unreasonable, what exactly constitutes a search or seizure, and the definition of probable cause.

Probable cause means that an investigating officer has enough evidence to make him or her reasonably suspicious of a particular person. If an officer has probable cause, she can testify before a judge and obtain a warrant to search the suspect’s home or other property for evidence. In cases where an officer witnesses a crime taking place, or sees a suspect fleeing the scene of a crime, a warrant is not necessary. In those instances, the police may search the suspect, their vehicle, and any surrounding areas in order to preserve evidence or ensure their own safety.

A particularly controversial subject is the definition of “search” itself. The Supreme Court has held that no search occurs if an officer views and seizes evidence from any place they are lawfully entitled to be. So, for instance, the police are permitted to stake out residences and businesses to watch for evidence of criminal activity, although the extent to which they can use technology to aid them is an ongoing question.

A due process right that is not explicitly guaranteed by the constitution, yet nonetheless has been granted by the courts, is the right to counsel when accused of a crime. It is not totally settled when exactly an accused individual has the right to an attorney, but it is clear that the right extends to well before the trial phase. The Supreme Court has ruled that at a minimum, when an investigation shifts from a general inquiry into an unsolved crime to a suspicion of a particular person, that person must be informed of their right to remain silent, and their right to an attorney.

If a court determines that one or more of a Defendant’s Constitutional Rights were violated during the investigation or arrest, the court will order any evidence collected as a result to be suppressed. This remedy exists to give legitimate meaning to the Constitution (it would be meaningless if there were no consequences for violating it), and to deter police misconduct. This is known as the “exclusionary rule”, and although extremely controversial, it is in effect in every jurisdiction. In addition, the exclusionary rule applies to any evidence that was gathered lawfully, but would not have been discovered but for the constitutional violation. This is referred to as the “fruit of the poisonous tree” doctrine.

If the investigation leads to an arrest and trial, the Constitution provides several more guarantees. The first of those is the right to a speedy trial, by an impartial jury. Many states have a statutory requirement that the prosecution bring a person to trial within a certain number of days, or the charges must be dismissed. A defendant may choose to waive this right in order to prepare his own defense, but the waiver must be knowing and intelligent, and the defendant must be aware of the consequences of doing so.

Other important rights involved in the criminal trial are the right to confront witnesses, to compel the testimony of favorable witnesses, and the right against self-incrimination. The Fifth Amendment right against self-incrimination actually applies to all phases of the criminal law process, though it is most commonly thought of in the context of a trial. This right also extends to statements made by the Defendant in confidence to certain trusted people, such as a spouse, priest, doctor, or lawyer.