Seals are tamper-indicating devices (TIDs) used to detect and report unauthorized entry, and perhaps discourage it. Unlike intrusion or burglar alarms, seals report unauthorized entry after the fact. They must be inspected, either manually or electronically, to determine if unauthorized access has taken place.

A seal does not need to physically resist entry. (In other words, a seal is not a lock.) Indeed, some seals are made of paper or plastic and can be easily removed or cut off. Certain security products, known as "barrier seals" do provide a physical barrier to entry. These are hybrid devices --part lock and part seal. Barrier seals should be used with care. A barrier seal is often a compromise product, neither optimum as a lock or a seal. It's dual function tends to complicate issues about how to best use the product.

The effectiveness of seals is strongly dependent on the proper protocols for using them. These protocols are the official and unofficial procedures used for seal procurement, storage, record keeping, installation, inspection, removal, disposal, reporting, interpreting findings, and training. With a good protocol, a modest seal can provide excellent security. On the other hand, a sophisticated seal used poorly may be worse than useless if naively trusted.

Seals are categorized as passive or active, depending on whether they contain electronics. Passive seals are typically intended for one-time use. Most cost from a few cents per unit to a few dollars per unit. Dynamic seals are usually 10-100 times more expensive, but can usually be re-used many times. In choosing a seal, it is important to bear in mind that unit cost is not always the most important economic factor associated with using a seal, nor is cost necessarily well correlated with the level of security a seal can provide.

Number of different types of tamper-indicating devices (security seals) analyzed and defeated by the Vulnerability Assessment Team to date: 244+
The table  below shows a summary of defeat statistics for 244 different tamper-indicating seals.  The seal attacks were undertaken by one individual, well practiced in the attack, using only readily available low-tech tools, materials, and supplies. The marginal cost of an attack is the cost to attack another seal of the same design by reusing the attack tools and supplies. See also Defeating Existing Tamper-Indicating Seals.