In criminal law, the right to a speedy trial can be used as either a procedural defense or a substantive defense in which a defendant argues that they should not be held criminally liable for actions which broke the law, because the prosecuting attorney failed to bring the case to trial for an "unreasonable" length of time.

In the US, the length of time can either be defined by statute (for example, in New York, the prosecution must be "ready for trial" within six months on all felonies except Murder, or the charges are dismissed by action of law, without regard to the merits of the case), or determined by a court under a substantive theory based on the 6th Constitutional Amendment; which states: "In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial..." This argument is typically made in cases in which a significant amount of time has lapsed between the date of the commission of the crime and the date of arrest.

It should be noted that most, if not all, statutes defining the period of speedy trial time also including various exceptions to this rule. Such exceptions typically include periods of time in which the delay preceding the trial is due to the request of the defense, or if there is good cause shown for the delay.