A custodian bank, or simply custodian, is a specialized financial institution responsible for providing securities services. It safeguards assets of asset managers, insurance companies, hedge funds, and is not engaged in "traditional" commercial or consumer/retail banking like lending. The role of a custodian in such a case would be to:

• hold in safekeeping assets/securities such as stocks, bonds, commodities such as precious metals and currency (cash), domestic and foreign
• arrange settlement of any purchases and sales and deliveries in/out of such securities and currency
• collect information on and income from such assets (dividends in the case of stocks/equities and coupons (interest payments) in the case of bonds) and administer related tax withholding documents and foreign tax reclamation
• administer voluntary and involuntary corporate actions on securities held such as stock dividends, stock splits, business combinations (mergers), tender offers, bond calls, etc.
• provide information on the securities and their issuers such as annual general meetings and related proxies
• maintain currency/cash bank accounts, effect deposits and withdrawals and manage other cash transactions
• perform foreign exchange transactions
• often perform additional services for particular clients such as mutual funds; examples include fund accounting, administration, legal, compliance and tax support services

Using US definitions, a person who owns street name securities and who is not a member of an exchange, holds the securities through a registration chain which involves one or more custodians. This is due to the perceived impracticality of registering traded securities in the name of each individual holder; instead, the custodian or custodians are registered as the holders and hold the securities in a fiduciary arrangement for the ultimate security holders. However, the ultimate security holders are still the legal owners of the securities. They are not merely beneficiaries of the custodian as a trustee. The custodian does not become at any point the owner of the securities, but is only a part of the registration chain linking the owners to the securities. Global securities safekeeping practices vary substantially with markets such as the UK, Australia and South Africa encouraging designated securities accounts in order to permit shareholder identification by companies.

The definition of "shareholder" is generally upheld by corporate law rather than securities law. One role of custodians (which may or may not be enforced by securities regulation) is to facilitate the exercise of share ownership rights, for example and processing dividends and other payments, corporate actions, the proceeds of a stock split or a reverse stock split, the ability to vote in the company's annual general meeting (AGM), information and reports sent from the company and so forth. The extent to which such services are offered are a function of the client agreement together with relevant market rules, regulations and laws.

Custodian banks are often referred to as global custodians if they safe keep assets for their clients in multiple jurisdictions around the world, using their own local branches or other local custodian banks ("sub-custodian" or "agent banks") with which they contract to be in their "global network" in each market to hold accounts for their respective clients. Assets held in such a manner are typically owned by larger institutional firms with a considerable number of investments such as banks, insurance companies, mutual funds, hedge funds and pension funds.