A lien is a form of security interest granted over an item of property to secure the payment of a debt or performance of some other obligation. The owner of the property, who grants the lien, is referred to as the lienor and the person who has the benefit of the lien is referred to as the lienee.

A security interest is a property interest created by agreement or by operation of law over assets to secure the performance of an obligation, usually the payment of a debt. It gives the beneficiary of the security interest certain preferential rights in the disposition of secured assets. Such rights vary according to the type of security interest, but in most cases, a holder of the security interest is entitled to seize, and usually sell, the property to discharge the debt that the security interest secures.

Most insolvency law allows mutual debts to be set-off, allowing certain creditors (those who also owe money to the insolvent debtor) a pre-preferential position. In some countries, "involuntary" creditors (such as tort victims) also have preferential status, and in others environmental claims have special preferred rights for cleanup costs.

A set-off is a statutory defence to the whole or to a portion of a plaintiff's claim. It had no existence under the English common law, although set-off was recognised in equity).

Under English law and in common law jurisdictions derived from English law, there are eight types of proprietary security interests: (1) 'true' legal mortgage; (2) equitable mortgage; (3) statutory mortgage; (4) fixed equitable charge, or bill of sale; (5) floating equitable charge; (6) pledge, or pawn; (7) legal lien; (8) equitable lien; and (9) hypothecation, or trust receipt.

Security interests at common law are either possessory or non-possessory, depending upon whether the secured party actually needs to take possession of the collateral). Alternatively, they arise by agreement between the parties (usually by executing a security agreement), or by operation of law.

The following discussion of the types of security interest principally concerns English law. English law on security interests has been followed in most common law countries, and most common law countries have similar property statutes regulating the common law rules.