A custodian is a Financial Institution that holds customers' securities for safekeeping so as to minimize the risk of their theft or loss. A custodian holds securities and other assets in electronic or physical form. Since they are responsible for the safety of assets and securities that may be worth hundreds of millions or even billions of dollars, custodians generally tend to be large and reputable firms.

In addition to holding securities for safekeeping, most custodians also offer other services, such as account administration, transaction settlements, the collection of dividends and interest payments, tax support, and foreign exchange. The fees charged by custodians vary, depending on the services that the client desires.

Notices must be supplied to customers when certain activities are conducted on their behalf or using their assets. Further, account statements must be supplied to the customers to keep them abreast of the current holdings associated with their assets.