Attachment is a legal mechanism for preventing a civil defendant from getting rid of property that may be necessary to satisfy a judgment.

When real property is attached, the attachment takes the form of a lien on the property, which "clouds the title" and prevents the defendant from selling the property.

Personal property can also be attached in one of two ways. If the defendant physically possesses the property, like a car or a stock certificate, the sheriff can take the property away and impound it until the matter is settled. If someone else physically possesses the property, like a bank possessing the money in a bank account, the sheriff can issue an order to the possessor to hand over the property until the matter is settled. (This latter process is also known as garnishment and is frequently used to squeeze money out of people who haven't been paying child support.)

If the plaintiff ultimately wins the case, the attached assets will be sold, and the proceeds used to pay down the plaintiff's claim. If the defendant ultimately wins, the sheriff returns the attached assets, and the defendant may be entitled to claim his legal fees and other expenses from the plaintiff (as well as additional damages for mental distress if the plaintiff acted with malice).

In the United States, attachment is a matter of state law, but it must conform to the requirements of due process found in the Constitution. In the 1991 case of Connecticut v. Doehr, the Supreme Court held that a state attachment statute must at least provide for:

  1. notice to the defendant (which may be prior to or at the same time as the attachment),
  2. a hearing with the defendant shortly after the attachment,
  3. that the plaintiff post a bond to protect the defendant in case the attachment is found to be improper, and
  4. that attachment not be granted absent "extraordinary circumstances" that indicate a "heightened threat to the plaintiff's interests" in the property.

(Cf. Fuentes v. Shevin, a similar case dealing with the provisional remedy of replevin.)

This last requirement is interpreted to allow attachment when a plaintiff's claim will probably succeed "on the merits," and when it appears that the claim may not be satisfied unless the defendant's assets are locked down immediately. Judges generally have great discretion to determine whether such circumstances exist.

One common basis for determining the risk to the plaintiff is by showing, through some sort of evidence, that the defendant is defrauding (or is about to defraud) their creditors by hiding assets, removing assets from the state or encumbering assets (for instance, by obtaining a mortgage on their house).

Attachment can also be used against a defendant who resides outside the personal jurisdiction of the court but owns real property within the court's jurisdiction. This gives the court "quasi in rem" jurisdiction so long as the property has some relationship with the claim being asserted.