A legal waiver is the voluntary surrender of one's known rights under the law. "Known rights" is the key issue for courts when evaluating specific causes of action. Written waivers must inform the signatory of which specific rights they are relinquishing for the documents to be enforceable. This follows the legal principle that someone cannot waive rights he didn't know he had.
When a criminal suspect is arrested in the United States, the police must inform the defendant of his Miranda rights prior to questioning. This warning includes the right to remain silent, the right not to self-incriminate, and the right to have an attorney present. Should the defendant answer questions voluntarily or confess to the crime, he is considered to have waived his Fifth Amendment right not to incriminate himself. The choice, however, must be freely made by the defendant and not forced or coerced. The court must also be convinced that the accused fully understood the rights he was forfeiting.
The right being disclaimed can be statutory or contractual; the waiver may be express or implied. A written legal waiver is frequently required before a person is allowed to participate in a high-risk sport or potentially dangerous activity of some kind. Parents of school-age children are often asked to sign waivers for school field trips. This is done to protect the school or facility owner from accident and injury liability.
A legal waiver does not always have to be written. In some instances, an individual's actions may indicate waiver by conduct. Examples include not bringing a lawsuit against a defendant within the statute of limitations period. If a plaintiff waits too long and fails to file suit before the statute of limitations expires, the law presumes that by taking no action, the plaintiff has voluntarily waived his or her right to sue. In criminal cases, a legal waiver by conduct may be implied if a defendant voluntarily takes the witness stand.
Courts have historically recognized that a criminal defendant's silence should never be in itself considered evidence of guilt, nor a waiver of any legal rights. In the landmark 1972 case of Barker v. Wingo (407 U. S. 514), the U.S. Supreme Court opined: "Courts should indulge every reasonable presumption against waiver," "they should not presume acquiescence in the loss of fundamental rights," and "presuming waiver from a silent record is impermissible."
A legal waiver may be something as simple as signing for a parcel delivery. When a package is delivered to and accepted by the recipient, the mail carrier is then absolved of any further liability or responsibility for what happens to the parcel after the receipt was signed.