Criminal liability generally requires both a guilty act and a guilty mind, expressed in the Latin terms actus reus and mens rea. Actus reus refers to the physical act or unlawful omission that constitutes a crime and must be voluntary to satisfy this element. Mens rea describes the requisite mental state, with different crimes requiring different levels of intent such as purposely, knowingly, recklessly, or negligently, as outlined in the Model Penal Code. Together, these two elements form the foundation for criminal responsibility. Crimes are categorized by severity: felonies are serious offenses such as murder or robbery, typically punishable by more than one year in prison, while misdemeanors are less serious offenses such as petty theft, generally punishable by less than a year in jail, fines, or community service. Specific crimes include larceny, the unlawful taking and carrying away of another's personal property with intent to permanently deprive the owner, distinguished from robbery, which involves force or threats, and burglary, which involves unlawful entry.
Criminal procedure follows a structured sequence designed to protect the rights of the accused. Felony prosecutions typically begin with an indictment, a formal charge issued by a grand jury stating that sufficient evidence exists to bring the defendant to trial, as required by the 5th Amendment for federal felony cases. At the arraignment, the defendant is formally read the charges and asked to enter a plea of guilty, not guilty, or no contest, the latter being nolo contendere. Many cases never reach trial because the prosecution and defense negotiate a plea bargain, in which the defendant agrees to plead guilty to a lesser charge or accept a reduced sentence. Defendants unable to remain free while awaiting trial may post bail, a sum of money or property that guarantees their return to court, although the 8th Amendment prohibits excessive bail. At the conclusion of a trial, the jury returns a verdict of guilty or not guilty, which becomes the formal decision of the case. The recurring tendency of convicted criminals to reoffend is known as recidivism, measured through rearrest, reconviction, or reincarceration rates that help evaluate rehabilitation and sentencing policies.
The Constitution imposes important constraints on how law enforcement investigates and prosecutes crimes. Police must have probable cause, a reasonable basis for believing a crime has been committed or that evidence will be found in a specific location, to make an arrest, conduct a search, or obtain a warrant under the 4th Amendment. Before interrogation of a suspect in custody, officers must provide a Miranda warning, informing the individual of the right to remain silent, that anything said can be used against them, the right to an attorney, and the right to a court-appointed attorney if indigent. Under the exclusionary rule, established in Mapp v. Ohio, evidence obtained through unconstitutional means cannot be used in court, serving to deter police misconduct. Prosecutors are also constitutionally required under Brady v. Maryland to disclose all material exculpatory evidence, evidence favorable to the defendant that may prove innocence or reduce culpability. Perjury, the crime of intentionally lying under oath in a judicial proceeding, undermines the integrity of the justice system and is punishable by fines and imprisonment.