A lien is a claim or hold upon the property of another as security for a debt or claim. Statutes in many states give so-called mechanics' liens upon land and buildings to various persons who have done work in constructing or repairing the latter, although it be only a contractor, and not the landowner, who is liable upon the contract. In a few states these statutes have been held to extend to architects; while in some other states it has been held that architects are not entitled to liens for services under these statutes.

These laws are of importance to owners inasmuch as they render their property liable for the debts of the contractors, since an unpaid workman, hired by the contractor, can secure his wages through his lien on the building. It is therefore common in building contracts to guard against loss on this score by a provision that the last payment shall be made only after the lapse of a certain time after the completion of the work. The period named corresponds with the limit of time for claiming liens. Workmen and sub-contractors are usually very familiar with the law in the details in which it immediately affects their rights, and commonly conduct themselves in their work, in cases in which trouble is anticipated, with a view to helping their legal remedies. Thus it will be found that sub-contractors very frequently prolong their work on one pretext and another, in the hope of extending the time within which liens may be secured. The legal means necessary to establish these liens cannot be stated here. The services of a lawyer are necessary, as in instituting any other legal proceeding.