Underwriting (UW) services are provided by some large financial institutions, such as banks, insurance companies and investment houses, whereby they guarantee payment in case of damage or financial loss and accept the financial risk for liability arising from such guarantee. An underwriting arrangement may be created in a number of situations including insurance, issues of security in a public offering, and bank lending, among others. The person or institution that agrees to sell a minimum number of securities of the company for commission is called the underwriter.

The term "underwriting" derives from the Lloyd's of London insurance market. Financial backers (or risk takers), who would accept some of the risk on a given venture (historically a sea voyage with associated risks of shipwreck) in exchange for a premium, would literally write their names under the risk information that was written on a Lloyd's slip created for this purpose.

In the financial primary market, securities underwriting is the process by which investment banks raise investment capital from buyers on behalf of corporations and governments by issuing securities (such as stocks or bonds). As an underwriter, the investment bank guarantees a price for these securities, facilitates the issuance of the securities, and then sells them to the public (or retains them for their own proprietary account). This process is often seen in initial public offerings (IPOs), where investment banks help a corporation raise funds from the public. The underwriter is obligated to purchase the entire issue at a predetermined price before reselling the securities in the market. Should they not be able to find buyers, they will have to hold some securities themselves. To reduce the risk, they may form a syndicate with other investment banks. Each bank will buy a portion of the security issue, and typically resell securities from that portion to the public. Underwriters make their profit from the price difference (called "underwriting spread") between the price they pay the issuer and what they collect from buyers or from broker-dealers who buy portions of the offering.

The services provided in the process of underwriting include:

Once the underwriting agreement is struck, the underwriter bears the risk of being unable to sell the underlying securities, and the cost of holding them on its books until such time in the future that they may be favorably sold.

If the instrument is desirable, the underwriter and the securities issuer may choose to enter into an exclusivity agreement. In exchange for a higher price paid upfront to the issuer, or other favorable terms, the issuer may agree to make the underwriter the exclusive agent for the initial sale of the securities instrument. That is, even though third-party buyers might approach the issuer directly to buy, the issuer agrees to sell exclusively through the underwriter.

In summary, the securities issuer gets cash up front, access to the contacts and sales channels of the underwriter, and is insulated from the market risk of being unable to sell the securities at a good price. The underwriter receives a profit from the markup, plus the possibility of an exclusive sales agreement.

Also, if the securities are priced significantly below market price (as is often the custom), the underwriter also curries favor with powerful customers by granting them an immediate profit (see flipping), perhaps in a quid pro quo. This practice, which is typically justified as the reward for the underwriter for taking on the market risk, is occasionally criticized as unethical, such as the allegations that investment banker Frank Quattrone acted improperly in doling out hot IPO stock during the dot-com bubble.