The Due Process clauses in the United States Constitution prohibit courts from exercising personal jurisdiction over a defendant unless the defendant has proper notice of the court's proceedings. To meet this rule, courts require plaintiffs to arrange for defendants to be served with a court summons and a copy of the plaintiffs' complaint. These papers are collectively called process.
Typically, it is not enough to simply mail process to the defendant. The summons and complaint must be either given directly to defendants or left with a suitable person at their home or place of business. Service may usually be performed by any adult who is not a party to the lawsuit. Plaintiffs may hire professional process servers to serve defendants.
There are exceptions to these general rules. For example, the Federal Rules of Civil Procedure allow defendants to waive in-hand service of process. Defendants who do so get more time to compose their answer. Those who refuse must compensate the plaintiff for the costs of arranging for in-hand service. See Rule 4(d) of the Federal Rules of Civil Procedure.
Rules governing service of process vary by jurisdiction. See State Civil Procedure Rules.
See Civil Procedure